I'm afraid for decent LP to digital transfers one must forget those USB turntables. You really need to do research into proper hi-fi decks using sites like www.vinylengine.com and http://www.pinkfishmedia.net/forum/ as well as others.
The mistake MANY people make is using an average turntable, a cheap needle and then expecting miracle results by using software to 'fix' the noise and problems. This WILL NOT result in a decent sounding transfer. It's FAR better to get a very good deck and cartridge, into an average soundcard and do little to no digital tweaking to get the purest sound. Many people use shitty decks, and good soundcards expecting miracles - I'm afraid this won't work.
All turntables are not the same, and you need to spend money to get good sound from them unfortunately. At the VERY least - and we are talking a budget deck here - you need to look at something like a Rega P3 turntable and a Rega or Goldring cartridge. If you want better sound go on to something like a Michell Gyrodec, though those are considerably more expensive. When I did the original 7FN transfers I used a Garrard 301 which is a 50s deck and is still considered one of the very best even today once serviced. To get a deck that good today you'd need to spend around £2000...
Of course one can go second-hand to get good sound - look at decks like Thorens TD150 or Technics SL-10 for reasonably priced superb decks, and ask on hi-fi forums about decent cartridges. LP is INCREDIBLY sensitive and is not like CD. Pops, clicks and rumble CAN be almost non existant with a decent deck and cartridge (and a clean record of course!). Unfortunately many cheap decks emphasise the faults of vinyl. You need the most expensive deck you can possibly get to get decent sound. It needs to be setup correctly too which means decent support shelves which stop vibration getting to the sensitive needle. Cartridge aligned properly which is an artform in itself. Again this costs money. There is no cheap way around this unfortunately. People spend thousands on turntables to get decent sound.
Next you need a decent phono-stage (plugging a turntable into a normal input you'll get very faint sound. You need a special stage to correct this). Again cheap stages = cheap sound. You need to spend at least £50 as a bare minimum to get a decent sound. I use a Trichord Dino + and even this is considered by many to be a budget item...!
The last thing is the vinyl itself. You need to search for a mint pressing. And once you have one you need to get it professionally cleaned (Google is your friend here - google Keith Monks or Nitty Gritty cleaners). I was lucky to get a brand new, sealed pressing of Empire recently and I'll be using that for 7FN v.2 transfers at some point in the future. After buying 3 copies of Star Wars I have one I am happy with now and will be using that also.
There really is no substitute for a decent deck, and I hope the above has been of some help. Lastly, here's a few links of decent turntable manufacturers with decks that don't cost tooooo much:
http://www.rega.co.uk/index2.htm
http://www.project-audio.com/
http://www.hi-fiworld.co.uk/hfw/featureshtml/turntableroundup.html
http://www.decks.co.uk/products/technics/sl1200
Remember: Deck first, Arm/cartridge second, Phono-stage third, digital processing (if any) LAST! Not the other way around. Digital trickery cannot be a substitute for the original sound with a decent turntable...
Happy spinning!