I don't think you will get the performance you are looking for out of 2 8800GTSs in SLI. They are old cards and won't perform well in today's games. That motherboard will not run SLI anyway. Yes the new chipsets do, but not that board. I would sell your card and buy a newer one to be honest.
I would suggest the HD 7770. That will fit your system nicely and its pretty affordable. Have you overclocked your system? I suggest you do if you haven't, that will give you some more fps in games and let the gpu breathe a little better. There are lots of articles on the web for clocking that cpu as it was very popular in its day.
Well yeah i tryed and got to 2.88ghz but with old RAM which was 400mhz instead 800. Will try again, i hope i reach 3.0ghz !!! Prob is the MB has it's tricks ... now i just updated BIOS to latest 3002 ver. Will see what can i do ...
Anyways for GPU i was thinking MSI6870 HAWK but its very hard to get here in sLOVEnia =)