Operating System Choice for Gaming

I am building a new system fairly soon and obviously will require an operating system to power it. I am going to be using this build for gaming. Should I use my copy of XP Home 32 Bit, a copy of Windows 7 RC 64-Bit until Windows 7 arrives, or jump on Vista. Is the Windows 7 RC worth using for the moment until the full 7 arrives?
My system specs will be as follows:
Phenom II X2 550
Sapphire Radeon HD 4850
4 GB DDR3 1333
Additional details/comments:
Games played are going to include Source games Left 4 Dead, TF2, Half Life 2, etc. Epic games Gears of War, Unreal Tournament 3. Crytek games Far Cry, Crysis.

Whatever you choose you need to use a 64bit OS to use that 4gb of ram. My build is going to be similar to yours too. For me I'm probably going to use Vista and wait to see more reviews about Windows 7.

Win 7 should be your final OS by the end of Oct. For gaming, the advantages are stability and DX11. I say stability cuz Fallout 3 (for illustration only, I know its not on your list) crashes like mad in XP for me. But is almost flawless in Win 7 RC. There is a noticeable performance boost too with Win 7 RC. But what to go with now can only be answered by the hoops you feel like jumping through with your OS in the next month.

If you choose Win 7 RC or your copy of XP, you will have to do a clean install of the final Win 7 release when you get it in your hands. There is no upgrade utility for either of these. If you don't want to do a clean install twice in a month, you should get the Vista w/ free Win 7 upgrade. Vista will upgrade to Win 7 final without a clean install.

Don't listen to the 32bit haters either. Especially in XP, for gaming, you will never come close to needing the extra .75gb that a 64 bit OS can offer unless you're running 5 intensive programs while the games are running. If you're doing that on a dual core, the ram won't be your bottleneck, the PEBKAC (Problem Exist Between Keyboard And Chair)

I'd personally choose whichever is cheapest to get Win 7. If it happens to be Vista w/ tech guarantee, then buy that. I'd probably go with XP still for now instead of having to figure out Vista only to abandon it in a month or try Win 7 RC now. I'd only do the RC though if I had access to a clean copy downloaded from MS itself. Getting it off a torrent is very risky as they are full of viruses and malware.

@Kevin: I'd actually recommend doing a fresh/clean install of windows 7. I've always seen performance increases with doing clean vs upgraded installs. Also, this close to windows 7 being fully released there is no reason one cant reinstall the programs/drivers and put random data accrued on a external for ease of installation.

thats a good idea unless you arent interested in clean installing every 6 months. but if you dont mind, thats a good idea