Normal game performance for this machine?

Hello,
I bought a MSI 4850 and I'm a little disappointed with the general performance of my PC in games.
In 3DMark 06 (1024x768, all others settings default) my PC score is only 8474 (SM 2.0: 3508, HDR SM3.0: 4740, CPU: 1687). Anyway, with X1550, my system score was only 2000!
Now at "real" benchmark.
The performance of AITD 5 in 1024*768 and highest quality is acceptable, about 20 FPS average.
In the same resolution, Crysis Warhead (with all settings in Mainstream, except post processing set at lowest and Shaders set to Gamer) the average FPS is 25. In general, it feel smooth except in some parts were it freeze for some seconds.
Finally, in TimeShift, in highest quality and antialiasing, the average FPS is 60.
So, my question is: Is this a normal performance for my system? Considering the video card, processor, etc.
Will be the FPS higher if I put a 5000+ oced to 6000+ or something like that?
I think that here the bottleneck is my processor.

System details:
- MSI R4850 512 DDR3
- AMD Athlon X2 3600+ @2.5 (oced to 4800+)
- 2 GB DDR2 RAM Kingston 667 @511
- MSI K9N4 SLI (Unfortunately the PCIE speedof this mobo is 8x, but I think main bottleneck is processor, not the x8 speed...)
- Cooler Master Real Power Pro 550 Watt
- Windows Vista Ultimate Service Pack 1 x64

Hi,
Thanks for your reply.
My RAM run at 667 MHz, however I made a asynchronous overclock because my PC run better at 2,5 and 501 than 2,1 and 737 in memory.
So, you think that with more RAM my game experience will improve more than with a new processor?

I've formated the PC about three times. I tested with XP and Vista. In XP games run better but there's no much difference.
What's the problem? Maybe the video card is defective?
I've latest drivers for video card, chipset and sound.