Well aside from the fact that that processor doesnt even exist, if its a core 2 its a quad not a duo, if it is a duo its not a Q9300 nor is it stock at 3.0GHz, i would suggest builiding it yourself. Its getting sold for a reason, and you have no way of knowing how it has been treated and what quality the PSU, motherboard and other parts are. And you can get a
The seller corrected that the CPU is Core2Quad (Q9300) and claimed that the system was perfect condition. Would this make a difference on your opinion?
Would I be able to build a better one if I build my own with the budget?
the Core2Quad stock speed is 2.5GHz not 3GHz which means he bumped up the FSB to 400MHz, there is no way to know if the voltage has been increased above safe levels or if it has been damaged by heat accidentally. Also building your own gives you more control over the parts, HP motherboards have locked out bios's which limits what you can do with it.
Just skimmed through some threads, and I found myself at the same problem earlier... I needed a better CPU (I have an Athlon 4400+, works... but that's it.) and I looked at new computers. For a real performance based computer with not much modding, you'd be looking in the thousands range, mainly because of the labor costs. Building it cuts out labor, so I would go with Hunter's link for a good bargain, I've yet to find a better deal than that.