According to the CPU support list the 65nm Conroe based Core2Duo line is the best CPU that board will support. I would look around for an E6600, I had one for a few years. Great chip in it's time. Really any of the Core2Duo line will be a huge upgrade from a Pentium 4.
I would not try the newer Pentium dual core 45nm Wolfdale based chips. That board has a Via chipset and I doubt the newer chips would work. You will also have a hard time overclocking with the Via chipset.