Graphics cards

I’m going off to college shortly and was looking at getting a laptop for school (well mostly for the portable games but don’t tell my parents), and was looking at the new Dell XPS’. However I was wondering if I could make an existing laptop that they offer up to par with the XPS instead of forking over the money for the new design. Well the base of the question is how well does the 128MB DDR ATI MOBILITY RADEON X300 PCI Express x16 Graphics card stand up to the 256MB NVIDIA GeForce Go 6800 Ultra.