FlightSimBoy nailed the whole 3.3gigs thing. Your lucky my bios only show's 2.85gigs in XP. Get Vista 64bit to use all your memory. As far as overclocking goes it really depends on the card. They all overclock differently. Check out the sticky thread on overclocking your gpu there are some others with 8800cards that have some clocks posted.
Mine is probally the highest overall on this forum that I know of for a 8800gts. I got a good card. I'm at 675/2016 ATI Tool would show 1008 for the memory cause it doubles like ddr2. I think a resonable expectation would be 600-625/950-975. Anything over that and you got a good card.
Download Riva Tuner that way you can turn your fan up to help cool the OC down some as well.
Also man you need Vista for Lost planet to use DX10. Your so missing out. If you pick it up Vista download driver 162.22 I have no Issue's with lost planet. Your screen name the same maybe i'll find you online sometime.