We're going to need a model number of the motherboard, just because it has LGA775 doesn't necessarily mean you can upgrade to Core 2 Duo, some 775 boards will only support single core CPUs, some boards will only support up to Pentium D, and many boards will only support some, not all of the Core 2 series. We need more information to recommend a CPU upgrade.
That board won't support anything in the Core 2 Duo family. The biggest, baddest processor it will support is a Pentium D 960 (3.6 GHz, Dual Core, 2x2MB Cache) which isn't a horrible chip. It'll actually outrun some of the lower end core 2 chips by a long shot.
Ha, this is the exact same thing I was thinking about doing.....I too have a Pentium 4, (I've grown kind of fond of it, though it is an almost universally maligned processor) in an LGA 775 socket. When I read that Core 2's fit into the same socket, I was ecstatic at first, thinking I could upgrade to a dual core, 64 bit system on the cheap, and without gutting my case. Then I started to read more.....and discovered that there was a lot more to CPU/mobo compatibility than the socket type. Then I decided, screw it, I'm a gonna build myself a brand new system, probably with an 1155 board, and i7-2600 processor! Unless the Bulldozer stacks up against an i7, not just i5, I am afraid that I am sticking with Intel.