I also can't quite figure out if it's worth my time to set one of my computers at mining bitcoins, how long should I expect it to take me to mine enough to register a domain?
Simply buy or trade for them. The easiest way to do that, in my opinion (from a US-centric perspective) is to set up a Dwolla account, transfer a bit of money from your bank to Dwolla. Then transfer that money from Dwolla to an exchange (Mtgox and TradeHill are two examples), and then buy bitcoins at market rate.
Then you can use them to buy goods and services (including namecoin names). Bitcoins are the "currency". Namecoins are sort-of related (using the same underlying technology), but are a separate product.
However, you can purchase namecoins with bitcoins using bitcoin-namecoin exchanges like this one: https://exchange.bitparking.com/main
Also, to answer your question, mining is really ineffective for home users. I mean, if you have energy to waste on it, that's fine, but you'll find it's more economical to simply buy the bitcoins through bitcoin exchanges (Tradehill, MtGox) than it is to mine them.