Well, I've been an eMachine owner for the past 12 years, and I will say that you'd be better off building an entirely new computer. It'd be a lot cheaper than trying to upgrade what you have now.
I still have an eMachine - The only original parts are the Pentium D Dual Core 3.0 gHz processor and the motherboard. All the specs are listed in my sig. I've spent a lot of money upgrading it to where it is now, and it runs pretty good. Next machine I get, though, I'm building.
But if you still want to upgrade, then we'd need to know more specifics. Example: motherboard model number, processor socket type, case form factor, etc..
It IS possible to make an eMachines rig a good one - I've done it five times in the past - but it is horrendously expensive.