The average 2.4C and 2.6C will top out somewhere in the 3.2 to 3.5GHz range. The main advantage of the 2.6C is that for any given clock speed you want to run at, the FSB will be lower, giving you a better chance at running 1:1 ratio and/or with tighter memory timings.
Example, say you are looking to run at 3.4GHz (provided you have a decent chip, this should be a possibility with good air cooling and slightly upped VCore). With the 2.4C you'd need an FSB of 283MHz, which means you'll almost definitely have to run at 3:2 ratio (unless you have PC4000), putting your RAM at a mere 183MHz. On the other hand, the 2.6C would require an FSB of only 261MHz, meaning you'd get to run at 5:4 ratio, giving you a RAM speed of 209MHz. Not to mention that you'd be stressing your board less with the lower FSB.
Whether those advantages are worth an extra $40 or so is up to you. I went for the 2.4C and am 110% satisfied running rock stable at 3.12GHz (260MHz FSB / 5:4 ratio) considering the chip was $170! Sure, if I had chosen the 2.6C and had received a halfway decent chip, I'd be running at 3.38GHz with these same settings, but I don't regret saving the $50... it's still plenty fast for anything I can throw at it and probably will be for at least a year or so.