They both make great cards, it just depends about what price range.
I say Overclock your card a little bit and see if you get an frame increase. That card will overclock a little bit at least as my 7900GS is at 615/835 on stock volts. Those Nvidia will overclock pretty well. Also overclock your E6600 a little bit, I mean geez you have good G.Skill RAM and a DS3. Also you have an Antec Nine Hundred case which has great cooling, it's like you bought your system for overclocking but changed your mind...Do a bit of OC and you should see a noticeable increase in performance.
E6300 w/ Arctic Freezer Pro 7 @ 3.6ghz 1.4Vcore
Antec 900 Case
EVGA 750i FTW Mobo
4x1GB of G.skill DDR2-800
2 8800GT in SLI
22" Samsung Syncmaster
1 250GB WD and 1 Seagate 7200.10 in RAID 0
Corsair 520HX 3DMark06 Score: 13900