Vista or XP for new (to me) rig

I am putting together a new/used rig:

Asus M2N-E (non SLI)
AMD X2 4600+
4 GB PC2 6400 (with potential to upgrade to 8 GB)
Nvidia 8600GT 512Mb GDDR3 (DX 10 capable)
500 Watt PSU
750 GB 250 GB HDDs both at 7200 rpms

I got a great deal on a used shell and will get out of this for under $400. It's a far cry from my Athlon XP 3000+ that I'm using now. I like to game - CS, DoD, BF2, COD 4, WiC and probably Mass Effect when it arrives to PC. I don't get crazy about my frame rate, as long as it's decent, I'm happy.

This system seems to be on the fence from what I've read about whether or not you should install Vista 64 bit (to utilize all of my RAM) or XP 32-bit SP3 (because XP 64-bit isn't compatible with many games, I know this from experience). Some people say if you have more than 2 GB of memory, get Vista. Some say if you have more than 4GB, get Vista. Some say if you don't have SLI, don't get Vista. It would be nice to use DX 10 since my Video card supports it. Also, I'm more than happey to throw some more RAM at it although I think my Processor is about maxed with 4 GB, then again, maybe not with Vista.

Any input? Anyone have a similar rig with Vista or who has tried Vista?

Thanks

My old computer slowed noticeably with 32-bit Vista and SLI-ed 8600 graphics cards when running "modern games," most notably "Hellgate: London" (No great loss there). That was with an even older AMD 2.0ghz processor, however. A 9600 or 8800GT graphics card would probably benefit you more than more RAM on that rig, regardless of whether you're using XP 32, Vista 32 or Vista 64.

I went with 64 bit Vista Ultimate and it is running very smoothly. I have experienced no compatibility issues with any of my hardware or software. DX10 actually does better on frame rate than DX9 in the game World in Conflict's benchmark program. Vista 64 is the way to go with any relatively new machine. Nay-sayers are just trying to relive the Windows 2000 fiasco but the information/evidence for why you should go Vista 64 is becoming more and more convincing every day.

I just finished a homebult machine with Vista 64 and was wondering the downside of installing XP other than loosing some memory.

Q6600 cpu , 4GB RAM