the PC version has better graphics, higher resolution etc but the game is currently very picky about what it will run well on eg with my pc
2800+barton, 1gig crucial 2700, fx5600ultra 256meg it runs like a dog (ie bad) with about 25fps
but with teh same cpu + ram and my old gcard (GF4ti4200 128) i was getin 50fps
it turns out its to do with the shaders used, halo was using 2.0 shaders with my FX and 1.5 with my GF4, hence the game looks stunning with the FX but suffers a performance hit, and looks good (still better than Xbox by miles) wi the GF4 but aint as good lookingas with the FX
luckly tehre are command line switches for halo (read its readme) that let you choose the sharders, so choose teh 1.7 shader for teh FX and then boy is it sweet