The 4890 will far out pace the GTX 275 in overclocking with core clocks sometimes reaching 1GHz. Also the 4890's memory is pretty overclockable with 1200 MHz being attainable for some. Stock vs Stock the 4890 and the GTX 275 are the same, but when overclocked the 4890 pulls away with its massive headroom.
I have seen reviews of the 275 having pretty good headroom as well. It is just not as hyped. But generally speaking the 4890 would be far easier to overclock on average. I can only speak of experience with the 4890, never owned a 275. My Oc was limited by the crossfire a tad.. But temps are only 1 or 2 deg higher than stock, I did not change the cooler.
Remember though, there is nothing sayign the card you get wont even be able to handle a 10% OC. Don't buy a card expecting the moon or you will be dissapointed.
It really depends on what you are going to use it for. Half the games get better fps with the ati the other half with the nvidia. If you plan on doing any folding@home you should defiantely get the nvidia card. If not then either card will more then do the job.
As close to being identical as you'll find. The only issue will be finding a 275gtx as they were never launched in huge numbers and I doubt they ever will be. The largest UK techsite has never had more than 2 different 275gtx manufacturers available since release.
On the other hand, be aware that ATI's are hotter, more power hungry and noisier than the equivalent nvidias that you can get hold of.
Yeah the GTX 275 can almost do a 100 Mhz core overclock and a 100 Mhz memory overclock like the 4890, but the 4890 gains more performance from it and can sometimes go higher. Both are great cards, but if you want to overclock a GPU then the 4890 is the way to go. Even with 100 MHz overclocks you are still comparing 740 Mhz to 950 Mhz, that is massive and Crysis seems to love that raw power with the core and is allowing the 4890 to dominate and even outpace the GTX 285 at times.
Don't get me wrong, either one is fine and they are both about the same I just got the impression the OP was interested in having some OC fun.