There are turntables out there with amp built into them. Both the Numark TTUSB (which I previously owned) and the Stanton T.55 USB (which I use now) are two very good tables and cheap (the Numark going for $99 and the Stanton for $199). There's also the portable Numark PT-01 USB (which I also own and use for when I go digging) which has a built in amp.
As Matt said, I've never used the USB feature on either of my turntables. Unless you're using it to archive and transfer you music to your computer for listening purposes, or MAYBE and you do your sampling with software (Reason, Fruity Loops, etc.) then I you'd be better off just going directly into you MPC or Fantom.