Well, this is really the only game I play, my system below runs the game at 1280x768 all settings on high with an average FPS of about 40-50. My system is pretty outdated, but you don't need much more than 30FPS to be able to play the game smoothly. There are (or were) people playing Halo PC on Pentium III systems; it's not a resource hog in the least bit.
To answer your question, 512mb should be plenty to play with, though a gig would be perfect.