When I was big into overclocking - had a brand spanking new (at the time) Pentium D 955XE...tossed it on the motherboard, hooked up my copper Mousepot (big copper tube) for some icewater testing, powered it up and POOF! Instantaneous burnt silicon...
Everything runs so much faster, but I haven't really focused on doing performance test for individual apps. I can tell you that Adobe Photoshop CS4 renders effects much faster than it ever did.
Sony Vegas Pro 8 also runs much smoother and faster.
As with most other people, I think you really notice the difference in Quad Core v. Single Core when you play system intensive multi-threaded games. Games such as GTA IV and newer Source Engine games run so much better.
I don't use any really demanding applications, i.e. video editing, database sorting, games, what have u. Consequently, the quad core equipped machines here don't look or feel any better than my old single core CPU.
OTOH, running a scad of process...
i have seen an increase in fps in games like crysis ,unreal tournament 3 and nfs undercover ,the applications like video converting and un zipping or un raring becomes almost four times as fast as on a p4 ,also you can do heavy multi tasking toget...