I have six 400SCs. You can use any PC3200 memory, ECC or not. I've read you can even mix them (you lose the ECC capability) but I don't know that I've tested that. I'd just buy two 1 GB modules (these work) and put them in dual-channel mode.
I haven't looked lately, but last summer Frys had Crucial PC3200 DDR memory for about $12 per GB after rebate.