upgrading 2007 imac?

Last week, I discovered that apple is planning to release os 10.8, and it occurred to me that now would be a good time to make any hardware and/or software upgrades necessary to speed up that mac. currently, its running leopard with 1 GB of memory, but I was wondering what (if anything) I should do to improve performance... I purchased 4 GB of ram for it, but I'm thinking of getting a total of 6 GB instead. also, has anyone upgraded the OS to lion or snow leopard yet, and seen better results since then? thanks!