SharifGT: The 6600GT is much better than the FX5500, but it will struggle with Halo 2. It may manage the lower detail settings at low resolutions, but if you want decent performance, you'll want something better.
So the 6600GT might be a stretch but you'll probably be able to play 800 x 600 with low settings... just a guess though. It seems to be picky on what video cards it plays well with.
So if I were you I'd invest in something along the lines of a 7600GT/X1650XT or an X1950Pro/7900GS if you can spare the extra cash.
A friend of mine's laptop has an X1400 Mobile, which although slower than an 7600GT is probably about the same, or slightly faster than a 6600GT, and that can't play Halo 2 properly at any setting. The frame rate dips as low as 10 sometimes.
