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.