Need advice for an upgrade

Hi guys I'm new to the forums and looking for some help. I recently picked up CoD4 and realized that my vid card can only run it at the lowest resolutions. Anyway I was planning on building an entirely new computer in fall or '08 so I just want to upgrade my graphics cards and CPU to play this game until I build my new comp. Below you will find my current PC specs and I just want to know what vid card upgrade would last me until I build my new PC.

Mobo: Asus A8N-SLI Deluxe Socket 939
CPU: AMD Athlon 64 3200+
Memory: 2GB Corsair
Vid Card: Radeon X700 Pro

Oh and I am willing to spend up to $400 on the video card. Thank you all in advance!

AMD Athlon 64 X2 4200+ Manchester 2.2GHz 2 x 512KB L2 Cache Socket 939 Processor - OEM

http://www.newegg.com/Product/Prod [...] 6819103543

XFX PVT88PYDF4 GeForce 8800GT 512MB 256-bit GDDR3 PCI Express 2.0 HDCP Ready SLI Supported Video Card - Retail

http://www.newegg.com/Product/Prod [...] 6814150252

If you can wait for the 8800GT to be back on stock (Don't know when) then get it. However if you can't wait, then either of these two would make a decent deal at their price point (The XT is the better deal):

SAPPHIRE 100201SR Radeon HD 2900XT 512MB 512-bit GDDR3 PCI Express x16 HDCP Ready CrossFire Supported Video Card - Retail

http://www.newegg.com/Product/Prod [...] 6814102095

SAPPHIRE 100212SR Radeon HD 2900PRO 512MB 512-bit GDDR3 PCI Express x16 HDCP Ready CrossFire Supported Video Card -

http://www.newegg.com/Product/Prod [...] 6814102706