Cheapest I know is either eBay, pricewatch, newegg. :hmm:
http://cgi.ebay.com/Gecube-X1950-Pro...QQcmdZViewItem
Most of them are 256 MB. I rarely found any 512 MB's on ebay.
This is the computer I bought from firstpacificorp
http://cgi.ebay.com/ws/eBayISAPI.dll...2BSI%26otn%3D3