i have the exact same setup as you and i can turn every high along with 4xAA and 16xAF without feeling any lag on cs:s (dunno about how HL2 is, but i'm guessing it's probably the same)
the only way i can ever lag on cs:s is if my crappy adelphia cable has a lag spike
btw, if you have $200, the upgrade to 6800gt is definitely the way to go, since 2x6600gt in SLI isn't very goodfor the price/comparable to 6800gt