If you were to build a system, the extra $30 or so will be definitely worth it to go DDR. You will notice alot better improvement in applications that require high memory bandwidth now, and into the future.
You can also find cheap deals on DDR mobos now too though so you don't have to get the best mobo out on the market. The ECS board is a good alternative as as Athlon4All stated, it has both SDR and DDR ram. The Duron 1G and the ECS board could be had for under $100 which would make a very affordable machine.