I used to own an Inspiron 3800 and the only way to add a NIC is to buy a PC Card version of one. It's simple... you just plug it into a free PC Card and Windows 2K/XP will ask you for drivers (or it may already have them and install it automatically). They're pretty inexpensive too. You can take a look through the For Sale/For Trade forum, or you can browse around on eBay. You can easily find one for $10-$25. I'd recommend a 3Com PC Card with an xjack; the xjack is nice since it retracts into the card and you don't need to carry around a separate dongle to hook up to a network.
Most newer laptops have what's called a mini PCI slot. You can install a mini PCI network card in these laptops and just use the ports built-in to the laptop. You need to open the laptop to install one, but you won't have a PC Card taking up a slot. Unfortunately this can't be done with an Inspiron 3800.