if you want to know which graphics RAM is faster
6750 have 128bit with 4600mhz DDR5 the memory bandwidth of this card is 134.4GB/s
GTS 250 have 256bit with 2000 DDR3 the memory bandwidth of this card is 128GB/s
so 6750 have faster memory bandwidth and a little faster on games and it support DX11
GTS250 support DX10 so it cant enable some DX11 games effects .