Hey man, you're really helping me out here, thanks! How much do upgrades usually run for? Give me some numbers and I can give you how much money I'm willing to spend.
Upgrading to the newest Core 2 Duo will be at least 160$, the
E8200, which has 2 cores at 2.66 GHz (compared to your 2 cores at 1.8 GHz)
The newest quad cores start at 200$, the
Q8200 and have 4 cores at 2.33 GHz.
It is important to note that two cores are sometimes better than four as software has to be able to use all four cores to get an actual boost in performance. Most games now typically use two cores, and many are not even optimized for that, so your best bet is probably to get as high a clock speed (GHz) rated dual core as you can. The middle of the road is 3.0 GHz, the
E8400, which is $170, and the best two you can possibly get are either the
E8600 with 2x3.33 GHz for 270$ or the
Q9650 with 4x3.0 GHz for 550$.
If your motherboard does not support the E8000 series, then you still have a number of options: the
E7300 with 2x2.66 GHz at 140$ or the
Q6700 with 4x2.66 GHz at 270$. Your last option is a
Pentium D 950 which will run about 150$ and has 2x3.4 GHz but only 800 MHz FSB speed, meaning it is nowhere near as good as the E8600 mentioned earlier even though it has a higher clock speed. The motherboard stats page at HP already indicates that you can use these, but the ones I mentioned before (the newer processors) are substantially better if you can use them, so I would recommend you call and find out.