I have a 965 AMD and a 2600 K Intel. The 2600 k is a benchmark monster and will clock up very close to 5 Ghz ( on water) with very little coaxing. My 965 will hit 4.0 Ghz with ease. With the differences in IPC and sheer clock speed you would think it would make a huge difference in everything you do . Well.... it really doesn't seem to make much difference for 90% of the things I do with my rigs .
Most games are pretty much the same when using the same video cards in both rigs ( however, I haven't tried both with extremely powerful cards and suspect the intel would be able to push them harder- just speculating). The game that shows the most difference is BFBC2.
I ran 3d 06 with both of them using an ati 5770 and the difference at stock was only a couple hundred points. Then I clocked the 2600 k up to 4.2 ghz to produce a score of 18146. If I remember correctly the 965 scored 17700's with the same card clocked at 4.0 ghz.
It seems if you are playing older games with a mid to lower range video card, there really isn't much of a difference. But if you play the latest games and have a beastly video card , the 2500/2600 would probably show its advantages. I would think the same would hold true for running later versions of futuremark's 3d benches.
If i were building a system from the ground up, and needed it today, i think i would go with the 2500 k . If i had the option to upgrade from a lower end dual core to a 955 or better quad or an X6 AMD chip on the same board and needed it today, I think that would be a viable option. But if i wasn't in a real big hurry , i would wait to see what bulldozer has to offer.