Questions about upgrading Mac OS?

Hi I have a Macintosh computer thats running on 10.4.11 and I was wondering if I should upgrade to leopard or snow leopard. Also will this upgrade increase speed? My Mac is really slow because its 7 years old and I was wondering if this upgrade will make it faster. Will the upgrade also give memory? I have about 20gb of leftover memory and I was hoping this OS upgrade will give more or none? Please help!