quick ram question

  Looking at buying a used older stick of ram to get some more use out of this computer. I have 256mb of ram, pc2100 ddr 266 made under the name NANYA, i believe.
  Ive been looking around on ebay and the prices are good but is it okay to mix sizes and brand names at all?

  I have a link below but i cannot tell what size it is so anyone want to identify it for me?

http://cgi.ebay.com/1024-Mb-184-PIN-DDR-DIMM-PC-2100-266Mhz-PC2100-1GB-REG_W0QQitemZ170049578193QQihZ007QQcategoryZ74942QQssPageNameZWDVWQQrdZ1QQcmdZViewItem?hash=item170049578193

>>6
Good point about RAM that is registered and uses ECC. I had no idea what that was until I purchased some memory with it, and my computer would not boot. Now I'm stuck with 512MB I can't use.