+1 on phono preamp. The Art DJ Pre2 is one of the most affordable quality options available: http://www.amazon.com/ART-II-Preampl.../dp/B000AJR482
You don't need to spend a lot on a preamp, and in fact, you'll see in the reviews that many people think the Art DJ Pre2 sounds better than $100+ models.
I'd stay away from USB turntables if you already have a nice turntable. I've had one, and in general for most models out there, the turntable quality is low, and the USB feature isn't all that helpful. You still have to play your LP's at 1x, and you may or may not have trouble with the USB drivers, so I wouldn't recommend it.
I use to run a Project Debut III w/ Art DJ Pre2, and I was very pleased with the sound quality, as well as track quality from analog dubbing onto my computer. For my setup, I ran the preamp output into a Mackie 4 chan mixer, and then ran the tape outputs of the mixer to my Soundblaster (was only a $30 Audigy card, but worked just fine).