perfomance wise a 9800GTX/GTS 250 is on par with a HD 4850 or better, so i wl say go wid nvidia coz of othr features such as CUDA n PHYSX......n moreover ATI drivers suck big time ...............one day or othr u r bound hv sm problems with drivers for sure........GO WITH NVIDIA n u wl b content.........
n plz dnt start war NVIDIA vs ATI. or 9800GTX+ vs HD 4850.......as i hv used both NVIDIA n ATI cards for smtime nw.........
but its ur call at d end of the day.......... basically HD 4850 is more dan a match for 9800GTX+ in sm games( loses in some games marginally) but its all about othr features in which NVIDIA wins hands dwn.............lol
regards........