OS Recommendation for Midrange Gaming Rig

I've finally bitten the bullet and moved ahead on building my first gaming system from scratch. Working with a $1000 budget here's what I've come up with:

* NZXT Tempest Black ATX Mid Tower Case
* Intel Quad Core 2 Quad Q6600 2.4 GHz LGA 775 Processor
* Gigabyte GA-EP45-DS3L LGA 775 ATX Motherboard
* G.Skill 4GB (2x2GB) 240-Pin DDR2 1066 SDRAM
* Western Digital Caviar Black 500GB 7200 RPM SATA 3.0Gb/s Hard Drive
* PNY XLR8 GeForce 9800 GX2 1GB Video Card
* OCZ GameXStream 700W ATX12V Power Supply
* LG 22x DVD Burner SATA Model Optical Drive

Now, given that this system has 4GB of RAM with another 1GB on the video card I'm guessing that the 32-bit versions of XP/Vista won't be sufficient to allow for full use of all that memory.

With that in mind does it make more sense to look at the XP Pro 64-bit version, which I'm guessing is much more stable, or the 64-bit version of Vista Home or Ultimate which probably have better driver and application support?

Also, if there's any other thoughts on the system that would help me tweak things feel free to throw them out there.

Thanks!

Get some DDR2 800 RAM that uses the standard 1.8V or you might have problems with the voltages required for that RAM you list.