For something cheap, and is good, AND tube-powered, the
ART Tube MP Preamp not only sounds good, but will bring your "line-in" to "line level," which is idea for recording.
I've seen them go for as little as $20 used (in Canada).
They're typically used for microphones as they provide phantom power for condenser mics, and the tube circuit gives a nice warm signal into your DAW.
You CAN plug a guitar straight into it as well, just be careful of the copious amounts of gain this little thing has. Great for acoustic/electrics even more so. In fact, I think I might pick one up for my new Washburn to see what the tracks sound like compared to straight into my mixer.
Behringer USB Mixers are reliable (in a Peavey kinda way), solid devices, and sound decent. If you can afford a tiny bit more, you could have that Xenyx 302 mixer. I've used Xenyx series USB mixers and they sound great. I'm planning on dumping my current 8-channel Yamaha USB mixer for the 12-channel Xenyx USB.
Are you just tracking one guitar (acoustic, electric?) and possibly one mic?