I would not buy the cpu in the hopes of taking it past 3ghz, I wouldn't even buy it in hopes with the 2.53. If you are going to buy the 2.26, just expect to get a couple of hundred overclock at most, and if you do get more, then it's a bonus but this way you don't let yourself down when it doesn't overclock as high.
You can look at it like this too. The 2.26 has the 533fsb so you would have to take it out of spec even higher (above 133fsb (x4)). If you got it to 150fsb (this is only a 17mhz overclock), then it would give you a 2.6Ghz machine but that's a bit much considering the chip is basically the same as the P4a chips but with a higher starting fsb so the scaling is not as great when overclocking.