To give more detail -- all else the same, the only difference you'll see is in stuff that's CPU bound like video encoding, super large spreadsheet calcs, numerical simulations, other engineering/CAD stuff, etc. Let's say something (processing a video clip to a different format) takes 30 minutes of 100% CPU on a 2.4GHz system. All else the same, a 2.66GHZ CPU system would do the same job in 27 minutes. The faster CPU saves you 3 minutes. If you do this all day long, maybe it's worth it to upgrade; if you do it once a month, probably not.
For most stuff we do; web, email, word processing, etc. the CPU is nearly idle that vast majority of time. Sure it may spike to 100% for ten seconds here, five seconds there. You'd never notice a 10% CPU speed increase in those instances.
So... it depends on your usage, but generally speaking if you're not hammering the CPU to 100% all day every day then a 10% CPU speed increase within the same family is unlikely to make a discernible difference. Spend your money instead on a faster hard disk or better yet a good SSD and you'll see much better performance for your money.