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You may not have to buy a USB turntable. For instance, you could buy some phono to 1/4 jack plug adaptors that would then allow you to use the soundcard on the Mac to get the signal in there. You would also a need another adaptor that you could put the 2 jacks into to make 1. You may encounter a bit of noise as this isn't exactly the cleanest signal.
It kind of depends on your long term view and what you're into. For instance, if you're into music production, you could kills 2 birds with one stone by buying an external soundcard e.g. if you got device such as the Firewire 1814by M-Audio, you could buy some phono to standard size jack plug adaptors and go into channels 1 and 2. You would need to pad to make sure the signal is balanced but I can testify that this definitely works, although there are other devices out there equally as good if not better. I highly recommend Mackie's stuff. M-Audio is OK, but their customer support is pretty flaky.
Re: the USB turntable, I have a neighbour who has a Seal turntable, and he is very happy with it. I had a play and the sound is reasonable. At the end of the day, it also depends on the quality of the software. If you use something like PEAK which is meant to be comparable to SoundForge, then you should be OK. Not sure whether GarageBand would be any use for this, as I've not really had a decent look at it yet.