From what i'we seen, gts250 1gb ddr3 IS faster than hd4770.
But the 4770 is cheaper, based on 40nm techology, has ddr5 memory.
So i'll tell my friend to get the HD4770
The GTS250 512mb, 9800GTX+/9800GTX, and 4850 are all faster than the 4770.
You can't really compare the DDR5 in the 4770 with the DDR3 in the GTS250.
Nvidia and ATI use completely different architecture.
All high end Nvidia cards use DDR3, whereas all high end ATI cards use DDR5. ATI cards also have 3x more stream processors.
Nonetheless, Nvidia cards still perform just as good if not better than ATI cards.
GTX260 DDR3 = 4870 DDR5
GTS250/9800GTX/GTX+ DDR3 = 4850 DDR5
GTX275 DDR3 = 4890 DDR5