Nights L is right, the K7S5A is the only one worth bothering with. It will bottle neck your machine but its definetely going to put your memory to work at its max.
There are lots of people with SDRAM laying around, many times at 512mb+ like yourself and I plan on putting my old PC133 to work the same way.
Its going to be a fast computer, at least as fast as a P4 with SDRAM as many had/have, but with an athlon you can use the latest CPUs all the way back to the socket a 700mhz model.
I've owned a ECS K7S5A in the past, very reliable board even in SDR, I used both SDR/DDR on it. The board is cheap also, probably $50 now.
Athlon 1700+, Epox 8RDA (NForce2), Maxtor Diamondmax Plus 9 80GB 8MB cache, 2x256mb Crucial PC2100 in Dual DDR, Geforce 3, Audigy, Z560s, MX500