Hmm, That doesn't prove much, tbh. With a quad core, windows would be sharing some of the work by default anyways, but seeing that one core running >90% tells me that it could well be the issue.
It's up to you really, mate - if you're willing to risk-it-for-a-biscuit, overclocking almost certainly would help - the other options are to buy a faster CPU or upgrade to Win 7.
The rest of the rig is definitely up to the task imo.
If you do decide to OC - read that beginners guide. Then read it again. Probably best to do it a third time before you even get into the BIOS. A friend who knows what he's doing on hand to help would be good - but as long as you take it slowly and try to
learn rather than blindly follow instructions, you should be fine
Feel free to PM me if you need any help, whichever way you go.