Avantgrrl
I'd stay away from e-bay you don't know what you're getting, anything else is fine but you shouldn't fool with memory (could be someone selling defective ram, or not have the right spec info). Your best option is to go with any popular online reseller using Kingston or Crucial memory, both sites sell generic memory. As a reseller I like Kingston because if product doesn't work, it's an easy return for my clients.
Memory shouldn't be too high though it's older technology so prices for this memory would probably be higher than a notebook memory 1gb that can run as low as $12 each due to supply and demand.
May be a bit pricey but the 512 is worth it, it serves a much bigger function on the Juno G than on a laptop or pc.