Probably get a new PSU while you are at it - some new motherboards and video cards have additonal power requirements above and beyond the older ones - and a PSU is usually one of the first items to go bad.
I'd look for a good 550 watt unit or higher. Really, most setups do not need the super-high wattages, and yours is going to fit into the "normal" category. Look for a quality unit buy CoolerMaster, PC Power and Cooling, or Antec. There are other good units as well, but those are well regarded and you can find something there to fit your needs. Look for a high efficiency (80% or more, if possible). Look at here for some more info to start:
http://www.anandtech.com/casecoolingpsus/showdoc.aspx?i=3516
Also, google some reviews or read the ones on NewEgg for more info.
You may want to consider moving up to the 4850 cards - looks like they are only a few $$$ more and are faster.
I have a Gigabyte board at home right now and have been very happy with it.