While there is a topic about this - I am getting a new Dell PC beast of a machine, and the grpahics card they offer is either the 256MB nVidia GeForce 6800 or 256MB ATI Radeon® X600. Which would be better? I know it would be better to get one with more RAM but I can't really afford it. The processor speed is 3ghz, and the overall RAM will be 1GB anyway.
I'd go for the x600, but that's just me. I'd also consider the nVIDIA 6600 GT. I think it's cheaper than both, though it only carries 128MB (don't know if there are any 256MB versions out yet).
Something Rainbow brought up: the PSU (Power Supply Unit). 350W should be enough for an ATI, if your PSU is NOT a no-name cheapo thing. nVIDIA cards are a bit more power demanding, but I think if your PSU is good enough it can cope with it (not higher-end cards though. Rule out any 6800!).
Ah yes, I just upgraded my RAM from 512 to 1GB. Loading times are significally reduced, game performance is slightly better though nothing to stand up and cheer about. Other specs: P4 Northwood 2.4 GHz processor, overclocked at 2.7GHz (I just love those Northwoods! Prescotts are no better than miniature barbeques!), Radeon 9200 256MB graphics card.