Upgrade GPU, how far can I go with what I got

[lokifalcon55]

I felt that the 7950 x 2 GPU was struggling playing some games now. I would upgrade but there is so much to consider now! I was looking at EVGA Geforce GTX 275 and HIS IceQ 4 Radeon HD 4870. Something in that price range. From my research I get the idea I would need a larger PSU if running the EVGA GPU. Also my MB max's out at 2G of RAM. I understand I cannot get a true 4G with 32 bit system but I cannot add anymore than 2 or MB malfunctions. Some of GPU requirements listed 2G of RAM as a minimum. Below is my system specs for any suggestions on what is a viable upgrade. How far can I go with what I got, If system will run noticable difference or do I need major upgrade like new PSU or upgrade MB? Will the ATI GPU work with Nividia MB? Do I need core i7?
thanks

Case Lian Li / Koolance PC4-1025BK
MB Asus P5N32-SLI Premium
CPU Intel Core 2 X6800 @ 2.93 GHz
GPU XFX Nividia GeForce 7950 x 2
RAM Kingston 2 x KVR667D2N5/1G
PSU Corsair HX620W
HD Western Digital VelociRaptor 150 GB, SATA 3 Gb/s, 16 MB Cache, 10,000 RPM
HD Seagate Barracuda 320 GB, SATA, 7200.10
CD/DVD Burner Sony DRU-V200S/BR DVD Rewritable Drive SATA
CD/DVD Burner Lite ON IHAP322098 SuperAllwrite Drive-IDE
Monitor Samsung SyncMaster 226BW 2ms 3000:1
Keyboard Razer Tarantula
Mouse Razer Copperhead
Sound Card Razer Barracuda AC-1 Gaming Audio Card
Speakers Bose Companion 3 multimedia speaker system
OS Windows XP Home

[flanzig1]

With the ram issues, are you adding another 2 sticks of ram for 4 sticks total? If so, may be better to install a pair of 2gig sticks.
With your system specs and the Corsair PSU, with the GTX 275 you could be pushing the limits of the PSU.

ATI video cards will work with Nvidia chipsets, just not in Sli.