32-bit vs 64-bit
The main advantage of 64-bit is that you can address more than 4gb of total address space. With 4gb ram, a 32-bit OS and a 512mb video card, the total system memory space would be 4gb, but that includes all the memory maps for all the hardware - primarily the 512mb of video card ram, but also there's maps for bios, soundcard, pci devices, disk I/O and indeed pretty much every device in the system. Therefore the amount of memory space which is real RAM in this setup would be something under 3.5 gb, and might be near to 3gb.
That's where the limitation comes from - you have 4gb minus the video ram minus a bit for other devices to map the system space.
NOTE: if your chipset doesn't support more than 4g ram it might have a 32-bit address space, rendering your 64-bit processor and 64-bit OS effectively a 32-bit one - a lot of laptops have this!
On the other hand, no program you're likely to run - at least for now - is 64-bit anyway, so any given program won't be able to map more than 2gb to it anyway - relevant if you have 8gb ram and expect your video editing suite to make use of it!
32-bit with more RAM?
P.A.E - most intel processors since the pentium pro support a 36-bit addressing system called Physical Address Extensions which can handle up to 64gb of ram, solving this problem. However.... it's a bit of a kludge, and also causes some drivers not to work properly - it's really best avoided. it also doesn't change programs, which in 32-bit windows will still be restricted to 2gb of ram for any one program.
4gb ram will give you 3.5gb or so under a 32-bit system, more if PAE happens to work for your hardware/driver setup.
Drivers - all drivers for hardware are written in 32-bit. Some manufacturers (for things like motherboards, almost all, for things like scanners/printers and older hardware, very few) have written drivers for 64-bit windows. therefore 32-bit windows XP has the best hardware compatibility by far, followed by 32-bit vista, followed by 64-bit vista, followed by 64-bit windows XP (yes, there is such a beastie - more on that later). If you want to plug in lots of odd or old hardware, 32-bit windows XP is better. for any modern Vista-compatible device in your system there will be vista drivers for 32 and 64 bit, and XP 32 bit drivers.
Conclusions:
64-bit works in Vista, and if you go for Vista then it's recommended. 32-bit is the best option if you go for XP.
32-bit means more driver support but means you can't build a particularly powerful computer, as you're stuck with 3Gb max RAM.
64-bit and Vista means everything works but some drivers won't exist (rarely, now) and some programs might not work properly (also rare).
Vista vs XP
Currently I'm recommending not to go with Vista for my general customers.
This is mainly as for any given computer (below something very expensive with 3+gb RAM, a high-end processor and a powerful video card) XP will be a lot faster than Vista, giving far better value for money.
Memory and Speed:
XP
XP has a typical memory footprint (the amount of system memory used normally when it's running) of around 250Mb. We recommend any system has over 350Mb as that allows some space for antivirus (Norton 70Mb, AVG 30Mb) and a program or two you might be running (Internet Explorer, Word etc).
As a practical Entry-Level system, I wouldn't fit less than 512Mb for XP, and normally 1Gb is fitted. Windows XP is a much smaller program than Vista and will therefore eat less of the system resources while you're doing something like gaming where the features of the Operating System matters less than how much CPU and memory it's using in the background.
Vista
Vista has a typical memory footprint of around 500mb, and It's common for an idling Vista system to be using 750Mb. This means the practical minimum RAM for a Vista system is 1gb, and we recommend 2Gb.
When you're gaming or doing something else where you don't see the interface of the operating system, with Vista you still have the cpu and memory load of the Aero interface (the transparent window effects), and of all the additional features running in the background. Therefore you need a much more powerful system to run a given program on Vista than you do on XP.
Program/Driver Support
Most hardware which is new works on Vista 64 and 32 bit. Older hardware is less likely to work, depending on manufacturer. For example, HP won't support anything older than 5 years on Vista - there go your old scanners and printers which still work fine!
On XP everything works as everything has to work on the largest market share OS out there.
Most programs work on Vista. That's MOST, not ALL. The older a program, the less likely it is to work - important if you like older games, for example! As time goes on, new programs will work on Vista, but for now everything works on XP, and most works on Vista.
Conclusions:
XP is by far the most compatible, for drivers and programs.
XP has by far the least memory and cpu hogging habits - you will have a much larger proportion of your computer's power available for a program than with Vista.
However - with over 3gb of ram, you are going to run into limitations of 32-bit XP. you could switch to 64-bit XP, but given that was never widely adopted the driver support is abysmal. 64-Bit Vista is probably the way to go - this supports most programs, most of the hardware you're likely to use, and a 64-bit computer can handle up to 2 Exabytes (But under Windows you're limited to 8 Terabytes, or 8196 Gb - still enough for now!)
The choices:
Your choices basically come down to 64-bit Vista or 32-bit XP.
Do you go for 64-bit Vista and just accept the degradation in performance in exchange for "better" looks and more features?
Do you go for 32-bit XP and just accept that you can't have more than 3gb RAM in exchange for more speed, and more reliable software?
You can go for 32-bit XP now and upgrade to Vista 64-bit when you feel like it (though this would result in you buying two licences).
My thoughts
Personally, without the very latest hardware I would run XP 32-bit as it's easier to fix, more reliable, works with more programs, and doesn't clog the computer so much.
My current laptop (AMD Turion TL-60 2ghz Dual Core, with 2gb ram) is very quick under Windows XP, and frustratingly sluggish under Vista - this shows just how much processing power Vista wastes! Shortly after buying this laptop I downgraded it to XP from Vista, which gave me a much faster laptop!