Your bottleneck is your video card, 8600GT was the mid range bang/for your buck card. You need at least an 8800GTS with 512MB to get that FPS up, or SLI your 8600GT. I have an 8800GTS with 768MB of video ram, and I don't know what my FPS is in L4D, but I play everything at maximum settings and never lag at all. I'm assuming your 8800GT has 512MB, and that's probably part of the problem. More v-ram = good. Also the 8600GT has less pixel pipelines for instruction, the higher end models have more instruction lines for code, which equates to better FPS.
