The pentium 4 is definitely holding you back big time. I used to have a 2.8Ghz P4 system, and the difference in both 3dmark05 and 06 are night and day. The Core 2's can handle soooo much more than the P4's.
You're going to have to upgrade your motherboard and memory if you get a Core 2 Duo. Your current motherboard only supports Pentium 4's. The video card is great (obviously). Just for reference, look at my 3dmark05 score. It's over 20,000 fully optimized (and overclocked). a good Core 2 duo and 8800 GTS 512mb will probably get around 15,000 without breaking too much of a sweat. The 8800 GTX is not that much more powerful than an 8800 GTS 512mb either. Your processor is a HUGE bottleneck to your video card.
If you have about $300-$400 to shell out on a new processor, motherboard, and ram, I'm pretty sure we can get you a nice deal from newegg.