Well, the 2500+ is probably your best bang for the buck. A 2600+ probably will overclock as high as a 2500+ will, but I doubt it would do more. The 2500+ is 1.83ghZ and the 2600+ is 1.9ghZ.
I'd probably go with the 2500+, or if you want an unlocked one, then a mobile 2500+ (or 2400+).