nickdgr8,
I think you've hit on the most common solutions on your own. The fact that you don't have a built-in NIC implies that you have an older system running perhaps an older version OS (I could be wrong). So you may need to consider the support your HW/SW will provide to any new HW you purchase. i.e. check to make sure that your HW/OS will support whatever you end up purchasing. There are really two major/common options available to you: Wired or Wireless. You can purchase two wired NICs or two wireless PCI NICs and get your machines connected via cross-over cable (for wired) or Peer-to-Peer wireless. These two solutions meet your simple "get connected" needs. If that's what you want then I would strongly recommend against wireless and for the wired solution as the wireless solution in this case is very insecure.
You can go a step further and get yourself either a switch (e.g. Linksys/Netgear/D-Link) that supports wired and/or wireless and connect your two computers via the switch which is what I recommend you do. If you opt for wireless then make sure that whatever you choose for wireless NICs and switch can support WPA2 so you can configure your wireless network to be the most secure. Also make sure that you go with support for "802.11g" not because of speed but because devices that support 802.11g will tend to provide better features. And since you don't have any NICs today that means that you are either using a friends computer to get on the internet or you maybe using dial-up. If you happen to want to move to the DSL or Cable world then you want to make sure that your purchased switch (wired or wireless) supports DSL/Cable. If I were to take a risk and recommend a product I would go with one of the many options found here:
http://www.linksys.com/servlet/Satel...VisitorWrapper for wireless or here
http://www.linksys.com/servlet/Satel...VisitorWrapper for wired.
Now having said that if you can live without wireless and wires aren't a problem for you I would personally stick with wired connections. There's less chance of someone hacking your network or you accidentally misconfiguring your wireless network. You also get roughly 4x the speed with a wired connection than you do with a wireless connection (in the case of 100Mbps wired vs. 802.11g). But if you're used to dial-up then either will seem fast

.
Good luck.
Casey