I made the mistake of letting my friend buy and E-Machine about three years ago (she didnt have a PC at all and wanted to e-mail) The first complaint she had was that it was so slow. I added a 512M stick to the existing and laughable 128M and it has run well that way through the other upgrades. In fact the only remaining E(vil)-machine parts are the AMD 1800+T-bred and the Monitor
I would not suggest that you mix RAM....although it may be the same speed, it may have different voltages, and timings which can lead to motherboard rejection .
If the Ram is different speed, it doesn't matter, because the Mobo will automatically clock it to Correct speeds. If you have a stick of PC2700, and a stick of PC3200, and the MOBO only supports 2700, it will down clock the PC3200 to PC2700 speeds.