DDR3 isn’t a huge necessity but there is a difference in performance and upgradability. Take into account how long you want to keep your system, if you think you’ll be getting a new one in a year or so then ddr2 will be ok. But if you want to take full advantage of your system and be able to make effecient upgrades later I would go with a DDR3 motherboard. It also depends on everything you’ll need it for, basically if you want faster access to your RAM its DDR3.
Hope that helps, I just built a PC system and I decided to go with DDR3 after all the research I did. But I don’t plan on ever buying a PC again, this should last for a while and then I’ll probably end up with a Mac pro somewhere down the road.
Oh and I also found with a 64 bit system its better to have the faster RAM. I have a brand new 64 bit system with CS3 master suite and my computer idles at 1.5G of RAM. With SD projects open in PS and AE it uses around 80% of my 6gigs. So with a x64 system and SD footage i would go with at least 6 gigs, at least. I’ll probably be upgrading to 9 or 12.