I would also highly recommend installing SP1. Not only will it get your USB 2.0 drivers working, but it will also give you a more up-to-date and stable system. SP1 contains lots of bug fixes and (especially) security patches that are crucial for any machine that is connected to the public internet.
I wouldn't recommend installing SP2 yet, since it's currently a pre-release build. You'll need to uninstall it anyway when the final build is released later this year.
The only difference between SP1 and SP1a is the MS JVM (Java Virtual Machine). If you install SP1, the MSJVM remains. If you install SP1a, MSJVM is removed, and you'll need to download the Sun Java runtime environment from http://java.sun.com in order to be able to view any Java-based content. It's your choice really.
You can use usb 2.0 without sp1 if you have a usb 2.0 controller card. The card will come with the drivers ..... I tell you though: You will knotice a huge diff in the speed if you hook it up to usb, as it is reverse compatible. It took me almost two days to back up 50 gig with regular usb and only about 3 hours with usb 2.0 ...