SLI performance wth 9600GT

Hi,

Yesterday I added a second graphic card to my system to get a gaming boost as i am planning to play some demanding games such as Crysis and GTAIV.

I did a quick performance test with 3DMark06 and was unimpressed by the improvement.
A single card gave me around 7400 (1440x900 settings) and two cards gave me a score of ~8500.

I then played Fallout 3 with medium to high settings as recommended by the game when i installed it. I played for an hour and it was very fluid with no slow downs but i want to know if there is a way i could get more out of my system...

First. are there tests/settings that i could do/use with some games that i have to see how much i can push my system? I own Fallout 3, Crysis, Prototype, Bioshock, GTAIV. And actually, how do you check the actual frame rate while playing on these games?

Second since i don't think i want to upgrade my MOBO at this stage I thought I could add 2 Gigs of RAM and/or a faster CPU (in which case do you think my PSU of 600 Watts would be enough).

I am limited by the board to AM2 socket processors (although i heard it may be able to accept AM2+) and DDR2 800 memory slots.

Would love to hear your opinions/suggestions!

Thanks!

My rig:
MOBO: ASUS M2N-SLI deluxe - nVidia nForce 560 SLI Chipset
Processor: AMD Athlon 64- Dual Core 4200+
GPU: 2x BFG GeForce 9600GT 512Mb (SLI)
RAM: 2x1Gb DDR2-800
PSU: Rosewill 600W SLI certified
OS: Window XP32bit, SP2
Monitor: 19" max rez 1440x900

You mean sell the CPU (not gpu) and get a better one?

Would I really benefit from the extra RAM with the games i play?

I heard safe overclocking can only give you minimal increases in performance.

as for cpu, there are people running phenom II's with the latest bios (though no official)
http://forums.guru3d.com/showthread.php?t=286970

dual 9600GT's would give performance to about a 4870

the PSU should be able to hold up considering my friend is running a rosewill 600w as well (though i would recommend a different one, im just saying it works)

an extra 2GB of ram would probably help a bit too