If you want to get the most of a dual channel board two are needed to enable the dual channel mode. Performance is gained with a pair of 1gb dimms over a pair of 512mb dimms especially in gaming due to what is called "memory holes" where the dimm is filled to see an overflow to the next rather then having more ram let on the same dimm. For multitasking a good look at boards that will also support the Opteron Socket 939 model cpus will allow for later upgrades.
I would jump to the ATI X1800 over the X1600 line for a single video card. But once you see a card like this and the remainder of your hardware selection is made an adequate power above 420w is recommended. One article on the X1800 points right to that.
http://www.diamondmm.com/X1800256PCI.php To have a little more power for multitasking along with the occasional gaming a good look at a slightly larger supply works.