Well i just bought a hd4890 and i am most impressed even though i am gaming @ 1440 x 900 res on a 19inch, i am planning on getting a 1080p 22inch when money allows, i think tbh the gtx 275 shades the hd4890 and they overclock about the same percentage but i read a review where the hd4890 overclocked to 1ghz on the core and 1200mhz on the memory (4800mhz).
The gtx260 is a great card and so is the hd4870 and i looked at these as well but i had a budget of £160ish so went for a hd4890 as the gtx275 was a bit over and also my mobo is crossfire as opposed to sli capable.
So i reckon the 4890 is a great card compared to the others.