Ok, so you bought an 8600, nice downgrade when you probably could have spent £10 on a new cooler and have a perfectly good card. But, hey... It's your money.
Wait, don't buy anything yet!
Faster motherboard? What do you have at the moment?
There's nothing wrong with PCI-e X16, it's the standard at the moment. Also, P4, that's socket LGA775, right? You could just get a new Core 2 Duo if you really wanted a new CPU.
I don't get why you're buying a new GPU, so you can buy a new MOBO, CPU and then a new GPU cooler. Why not just buy the cooler now, cut out the 8600! Ain't wort it dude!
Yeah, it makes more sense to just leave getting a new card, when there's nothing wrong with your current one, many people have a problem with the fan, getting a new cooler would fix it. Also, if you think the card is OC'ed, have you looked in Rivatuner to see if it is? Also, check out GPU-z, it'll tell you the stock speeds and the current speeds, so you'll be able to see if it actually is OC'ed.
No worries man! Hope this helps some!