If you're going to build a new system, definitely go with a DDR3 build. But first of all, what are the specs of the system you have right now? Maybe an upgrade will be enough to last you another few years instead of having to spend all the money on a new build.
My current system is a Dell Dimension E521 which I've already upgraded to an AMD Athlon 64 X2 2.8 GHz Windsor CPU, 4 GB of DDR2 667 MHz RAM and a 500 GB hard drive.
I don't play games that require a lot of power. Mostly what I play are Facebook games. What uses the most power is Dragon NaturallySpeaking speech recognition software which I use because I'm disabled. It tends to slow way down when issuing commands in Facebook games and some other software.