If that old P4 is $80 I'd rather get whole new combo, with some cheap P35/45 mobo and E5200 or something, even with single threaded apps it would be so much faster than that P4 (uses less electricity too)... and DDR2 is dirt cheap nowadays
there's no point upgrading the cpu in that board. a P4 is going to cost a fortune and yield very little improvement. just get a cheap-ish intel motherboard and get a pentium dual core.