Then there are various 4890's at stock GPU clock speeds.
Some important things that are also influencing my purchase are temperature levels (idle/load) and also how noisy the fan gets. I've read that with the XFX HD 4890 900Mhz gpu clock, the fan can get very noisy.
I guess the difficulty with all this is finding the right balance.
Thats more of a marketing gimmick if you ask me.. Go for a 4890 try a 950/1000 mhz core. reference is fine aswell easy to up the voltage and core speed on it, so either way is fine. Also why exactly are you upgrading your psu? since that psu has enough juice to power the 4890/275
They are both comparable in power. But which one will fit into your case? Depending on the alignment of the PCIE 16x card slot, a longer card might interfere with some of your components. So scope that out before you decide, because a 10.5 inch Nvidia card is a VERY tight fit.
I know, I've tried to put a 10.5 inch gtx 260 in my case. GTX 275 is the same length as the 260. I decided it was too long, so went with a 4890 vapor-x instead. It's an inch shorter and the vapor-x is apparently the quietest cooling solution for 4890 out there so far.