I'm pretty sure that the 915 chipset doesn't support dual-core cpus. The best you can upgrade to is something from the 600 series of pentium 4's. They have a 2MB cache and you can get something like a 650 that runs @ 3.4Ghz.
where do you find a P4 6xx CPU... I can't find one on pricewatch.com or any other website...
I just purchased an Intel Pentium 4 Extreme Edition 3.46GHz 1066MHz LGA775 CPU from StarMicro for $75. Apparently, it's the fastst processor they made for 915's. IT has 2MB of L3 and 512K of L2 cache. I'm waiting for it to arrive - hope it works.