In the event you can't locate your original documentation, you can check your computers exact hardware list from
http://support.dell.com Just put in your pc's Service Tag into the space on the site to retrieve your system details.
The Service Tag can usually be located under the lower panel on the front of the tower (it may lift up and expose USB or firewire ports depending on the case), and also on the back of the pc on a white tag under a barcode.
I also came to the same conclusion as Sigma about it likely being PC800. A quick search on Dell's site revealed 3 PC800 memory chips for the 8100, none were cheap. That link is
here . Unless your worried about warranty issues, I'd also go with newegg, though none of the currently in-stock stuff is particulary cheap either.
