Well, after a long and bitter argument between two audiophile friends - S is a venue sound tech/DJ while C is a session musician - they could, obviously, not agree on a damn thing except that I should probably look at using a pre-amp to my line in rather than going the USB route as USB will be more susceptible to noise caused by the power feed. This would work doubly well if I can talk Housemate into letting me use her computer while she's not here, as it turns out she has an extremely expensive pro sound card from when a job was flogging off old desktops and forgot quite what was in *that* one.
S was quite insistent that I should get a Shure SM58, whereas C said no, great for stage work, but far too sensitive for what I want, and would probably pick up the fan noise *anyway*. C has more experience in a studio situation, so I decided to listen to him. He said the one I mentioned above looked good, but doesn't come with a stand or anything and then there's the USB power-noise issue. So, he's suggested I get this kit (http://www.iwantoneofthose.com/podcast-studio-usb/index.html) from iWoot, and if it turns out the mic isn't as good as I want, he can probably find me something good second hand.