Give us some more information. As far as I know, all eMacs have hard drives with 2 MB buffers. However, some of the models (generally the higher end ones, and only in the more recent revisions) have 7200 RPM drives. Most have 5400 RPM drives.
An easy way to figure it out is to open Apple System Profiler ("About This Mac" in the Apple menu, then click "More Info") and look for the drive manufacturer and model number. Once you find it, look it up on the drive manufacturer's website - Seagate, Maxtor, Western Digital, etc should all have a full listing of drive model numbers and specifications.
For quick reference though, the eMacs that shipped with 7200 RPM hard drives are these:
1 GHz w/ Combo drive, 60 GB hard drive, 128 MB of RAM, dual boot, introduced May 2003
1 GHz w/ Superdrive introduced May 2003
1.25 GHz w/ Superdrive introduced April 2004
Any eMac purchased after the October 2003 price drop with a larger BTO hard drive