At my old job I was a lead Tech. I dealt with clients, did warranty repair and custom builds of systems for people. On the sales side of the company there were some gamer kids who swore up and down on high end over clocked memory like corsair.
I totally did a pepsi challenge with them. You see we would custom build a demo gaming rig so our clients could come in and witness the new hardware for sale and actually play games on it. We had UT and HL2 loaded on it, and I think Doom 3...
Anyways, the original build had corsair memory in it. I took 1 gig of the highest end DDR corsair memory and 1 gig of Patriot (real cheap memory) and ran the games once with the high end and once with the cheap.
No one could really tell the difference, a few people guess right but they had a 50/50 chance of guessing right anyways. Sure if I ran some benchmarks some tests came out a few points higher, but its not like it made a huge difference.
One thing I did see a lot building higher end systems is bottle necks, that would eventually lead to memory leaks and crashed applications after some time of useage (like a few hours).
That is why over clocking stuff is almost pointless. You put more strain on your hardware and get minimal real world performance boosts. Sure in benchmarks, and in some extreme cases you may get a noticeable difference, but in real world applications and even in gaming you probably won't notice a difference.