The latency you'll experience is largely down to your system...mainly the CPU speed and hard drive. With a relatively good machine you should be able to run at 11-22 ms, which is certainly usable. And yes, the guys on the Tascam BBS are very helpful.
The 428 and 224 are USB 1.1 compatible. While they're hardware-compatible with the USB 2.0 spec it would require a hardware mod to make them run at that speed.
Since you've already checked out the Tascam website, there are a few docs I suggest are crucial to download. One is the updated manual. The manual that comes in the US-428 box is pretty limited....I did a revision last year and the new version is much more detailed (like 80 pages vs 20). Also, download the PC optimization docs...there's one for W98 and ME, another for W2k and XP.
Hope that helps.