Ah, you didn't mention you were using 32 bit.
Having more RAM allows you to have more programs residing in your memory which reduces swapping. You may have a higher clock when OCing but that won't help you when you have memory-hogging apps (e.g. photoshop) swapping every now and then. There, it's your hard drive that will be the bottleneck. More RAM will make your programs snappier.
Figured it was obvious since I said I was upgrading the RAM when Windows 7 comes out. Should have mentioned.
As far as memory hogging apps, yeah, photoshop is about as bad as they get, and is a worst case scenario. For example there will be little swapping when 3D modelling/rendering/compiling.
Tatsujin - We all know you don't know anything about computers

Xtras - The mobo limit would be 24GB I think - with 6*4GB chips, but as I mentioned I don't want to fill all of the RAM slots as that significantly lowers the overclock, as the heat generated with all slots full and the overclock would smell like north bridge computer death. So if anything I could get 3*4GB for 12, but could use the price for 4GB chips to fall a bit before that.
Erious, nice to see someone who understands

...