LPs to .WAVs: 24/96 or 24/192 ?, USB drive OK ?

USB drives should work fine, but Firewire is a superior connection. The drawback of high sampling rates is that they take up a greater amount of disk space. 192kHz will require 4x the space as 44kHz. Don't forget that for safety reasons you'll also have to backup your data. The sonic benefits of recording at the higher rates depends upon the type of music you'll be recording. Most rock type music really doesn't benefit from anything more than 44kHz. Music with a wider dynamic range would be a better candidate for higher sampling rates.

Let me correct my above statement. Regarding what type of music would benefit from using higher sampling rates, it's more complex than dynamic range. Most pop/rock music is heavily processed and although not as a direct result it also has a limited dynamic range. My experience is that the heavily processed music really doesn't benefit from using the higher sampling rates. Possibly more important than the sampling rate is the bit depth. My experience is that all types of music benefit from using 24 bit A/D converters.

Gboren is correct, hard disk space is cheap, however, there are problems with installations containing multiple large hard disks drives. Unlike vinyl discs or even CDs, your hard disk drive will not last forever. Eventually, it will crash. You will need a method to back up your data. Paradoxically, the cheapest way to back up data is via added hard drives. Internal hard disk are convenient and don't generate the noise problems that external drives can. Unfortunately, most computer have only limited drive space and heat management (the major cause of drive failure) can become an issue.

Although somewhat pricey, the Lynx card is considered the best sounding computer card with built in converters. RME cards are less expensive, but they don't do 192kHz, only 96kHz.