4850 != 5750 (or <> if you prefer
)
The GPU is rather different, tho not completely different, and is made for DX11 rather than DX10. The memory controller of the 5750 is also optimized for a narrow 128-bit interface with GDDR5 while the 4850 has a wider 256-bit bus paired with GDDR3 memory. Also, the 5750 has some of it's sp disabled (as opposed to the 5770), while the 4850 has all 800 enabled.
As for the nVidia side, yes basically
8800GTS 512 = 9800GTX = 9800GTX+ = GTS250
While the first two are based on the G92 and the latter two the G92b, they are essentially the same chip with the biggest difference being the G92b is simply made at a smaller process and the clock speed they are set at. The 8800 Ultra tho is based on the older G80 chip which is why it has a wider bus (and uses a heck of alot more power).