If you’re considering buying an old eMac I’d recommend going for either the 1.25 GHz or 1.4GHz as both can be fitted with a maximum of 1GB of RAM. Despite the closed form, memory can be fitted easily via an access door underneath & with maximum RAM both models will run Mac OS X Panther or Tiger without any problems. Although Leopard should also be able to run on both of those models I would imagine it would be significantly slower, and therefore probably not advisable. Memory for all models of eMac can be purchased from Crucial.
One final thing to be aware of is that the eMac weighs a ton! So make sure you have a sturdy desk!