A basic usb input box will do just fine.
In my experience, USB interfaces tend to be much less reliable than FireWire because they depend on the CPU being able to rapidly respond to interrupts to copy large chunks of isochronous data into memory before the buffer overruns (which cause pops, crackles, etc.). FireWire, by contrast, does the most time-critical part of the heavy lifting in the host silicon itself, leaving the CPU free to do other things. FireWire cards are more expensive than USB cards because they basically have a small CPU on them....
USB-based recording interfaces can be usable for a small number of channels (4 inputs or fewer), but it depends on how the interrupts are shared on your motherboard. Folks sometimes find that they have to buy a separate USB card to make USB audio interfaces work correctly, at which point you might as well buy a FireWire card.
My advice would be this: if you buy a USB interface, just buy it from someone with a good return policy. It will either work out of the box with little effort or it will be a constant pain in the backside. If it doesn't work perfectly within ten minutes of opening the box, send it back and buy something FireWire-based. You'll be a lot happier.
Also, I would caution that the sound quality from USB interfaces tends to be poorer than that of FireWire interfaces, though there are exceptions (e.g. Edirol gets good marks in this regard from several people whose opinions I trust). For the most part, when companies build USB interfaces, it is a corner-cutting move because it is cheaper than FireWire. If they are severely cutting corners on the interface, you can generally assume they are cutting corners in the analog path as well. Just a word of warning. I've heard some pretty bad sounding USB interfaces over the years.
Finally, I would add that USB 2.0 fixes a few pretty serious flaws in the original USB audio spec. You're a bit better of with 2.0 interfaces than 1.x (though nowhere near as robust as FireWire). Some USB 1.x devices really sucked hard.
Caveat emptor, however: some early USB 2.0 audio interfaces used the USB 2.0 bus, but with an old-style USB 1.1 audio descriptor. These devices should be avoided at all costs because they aren't fully compatible with... well, anything. Don't ask me for model numbers, though; I got that tidbit from one of Apple's USB audio driver engineers; I haven't encountered those personally. Anything you buy new today should probably be fine in that regard.