Problem with my new 8800 GT

Hi all.
I recently purchased a XFX Nvidia 8800 GT 512 MB and I have some trouble with. Almost none of my games works with a number of FPS (Frames per second) resonance.
Crysis game plays a little pressure between 15 and 20 FPS, Bioshock Same near 15 FPS, all Source games like Half-Life 2, Counter-Strike Source, Team Fortress 2 is between 15 and 25 FPS which is really not normal. Even with my encienne Video A card ATI Radeon X700 PRO 256 MB all the games source operating between 40 and 60 FPS. ¸

My PC
Motherboard: ASUS P5LD2
Graphics card: XFX Nvidia 8800 GT
Memoire RAM : 2X 512 Mb DDR2 533
Processor: Pentium D 915 2.80 dual core

Helped me please
I am not an expert but to me after such a lack of RAM for this video card and you thought that you ?

Did you put the latest display drivers ? it is probably true that Vista on the desktop is 700 mb sometimes but often around 950 MB so if you calculated it is still not much, knowing that in vista games console decreases to 700 mb ram, you still 300 MB for crysis ! bioshock. So you will soon get another GB of ram.

It's quite effective, but 2 GB on Vista is not a luxury, but if it gave 40a 60 fps with the ATI card, I suspect again a problem with Vista drivers from nVidia, because many Internet users have already complained about it.

I tried my video card on another pc more powerful containing 2 GB of RAM and everything rolls like clockwork Crysis between 35 and 50 FPS with a high setting so I believe that these fine and well but my RAM its could also be the processor because the processor of the other PC is also much more powerful and have the latest driver from nvidia.

ok, if you have the right drivers, then the CPU or RAM, but it's weird because it worked well with your ATI X700.
you can add more RAM, but I think we need to change your particular CPU. What CPU to another PC?

Pentium 4, is quite old and is a little better than the P4, below the Core 2 Duo E4300 eg.Pentium 4, It did not limit the X700, which formed a balanced configuration with your Pentium 4. Your current configuration is very unbalanced, with a high-end graphics cards and CPU entry today.

I just tried 3DMark06 and when it made the test chart I am almost 25 fps on average, but when did the cpu test is not really super! I am not even 1 fps the image changes every 3 seconds. so now I was that my processor is old then that I should change it more than my memory.

So thank you for all your relevant information I intend to change everything in the near future.