I remember Emachines as having proprietary parts which means it's unlikely that you'll be able to drop in a standard part (That was a really annoying feature of OEMs back in the day.).
Also what OS do you intend to run ?
There are replacement parts specifically for Emachines however :
EMachine Parts
If all you're after is an inexpensive desktop for general use give us a budget and we'll put one together for you that you'll be able to upgrade in the future with minimal hassle .