Dude, you do know that the 9600GT KILLS in SLI, right? It scales like no other. After adding a second 9600GT to my set up, my 3Dmark06 score went from ~12,000 to 17,420 (click on the score in my signature for a compare chart in the FM ORB)!
That's an insane jump in performance for just adding a second mid-range graphics card! In fact that score bests even PCs running faster CPUs, more RAM (then me) and 9800GTXs!
As for the games, I play every game you mentioned with the exception of Crysis. So far I've run:
Bioshock, COD4, BF2, BF2142, Grid, AoC, Gears of War, Supreme Commander, The Orange Box, FEAR, DOOM3, UT3, CSS, and the latest iteration of MS Flight Simulator.
I play ALL of those games at their MAXIMUM settings (including maxed out AA/AF) at my screen's native resolution of 1680x1050. Even so, Im still getting a 60FPS average frame rate in pretty much all of them (via FRAPS and in-game monitoring). Not once do I experience choppiness, slowdown, skipped frames, or anything else which would detract from game play.
Given that your motherboard supports SLI, and that the 9600GT is going for a super cheap average of $135 USD, its well worth the investment for you to get a second one, IMO. That way you wont ever have to compromise on your graphics settings.
BTW, I DO run the Crysis Benchmark Tool. With that, I can run it smoothly at full detail, and at 1680x1050 but with AA turned off.
Plus, given that we have very similar set ups (I'm running a Q6600 too, but with half as much system RAM), I don't see why you cant/wont get similar performance figures. So it's not like I'm running a super cast CPU compared to yours or something.
BTW, since you currently own a 9600GT, this is a good read for you:
Keep in mind all of my testing and benchmarking was done BEFORE I found out about that "trick".