You'll notice in benchmarks. You'll notice in many, many games which don't yet take advantage of dual cores, never mind four. You'll notice in Crysis, which you can now play at 28 fps rather than 23 at high res & detail.
You'll also notice your ballistix and possibly northbridge frying. =) No, seriously, OC + 680i + ballistix = good chance of RMA sooner rather than later.
2.6 ghz is pretty much where a core2 CPU stops limiting a single 8800gt. If you find performance in things is poor, then overclock. Until then don't worry about it.