I'm guessing that you are on a budget, and are therefore looking for a budget solution.
If your current P4 is a socket 423 or 478, then it will be hard to find a new motherboard ; if it is a Socket LGA 775, then the hardware should be easily available. Just one point, 4 DDR slots would be better than 2 -- unless you have a micro-ATX box, and are therefore limited to a micro-ATX motherboard.
If you are absolutely in need of a new motherboard and have one of the older P4s for which no mobos are available, and already own your DDR2 and other basic components etc, then probably the best budget solution would be to get a AMD Athlon or entry-level Phenom ; the medium budget range AMD Athlon/Phenom X2 and Intel Core2 Duo DDR2-based dual core solutions are mostly comparable, although the triple-core Phenom II chips are quite attractive - the upgradeabilty of AMD-based hardware is better than the Intel equivalents, whilst the Intel chips tend to provide better clock rates ; of course, the upper budget range is massively dominated by Intel currently, mainly due to the high speed triple-channel DDR3 available with their Core i7 range of CPUs, DDR3 even being completely unusable with any current AMD-based motherboards.
I'm not sure why you wish to change your motherboard, but in any case you should try and aim for the best possible performance within your budgetary and technical constraints, which should probably involve changing both your motherboard and your CPU, at the very least...