The real issue with USB keyboards is that if multiple keys are pressed within one polling interval, the order is ignored. For fast typists, this can easily result in swapped letters. It's quite annoying. Unfortunately, this behavior is part of the HID spec so there's not much that can be done now.
To the submitter: I'm very sensitive to input lag, but I've never had noticeable lag from any PS2 keyboard. Right now I'm using a Dell AT101W, and before that I had some junky IBM membrane thing. Do you have any software installed for e.g. multimedia keys on the keyboard? That's the only thing I could see causing a problem unless the keyboard itself is just bad. Not sure I'd trust reaction time sites, though -- that's a pretty coarse measurement. If it's just for gaming, I wouldn't worry about the keyboard. The hand-eye coordination for the mouse is where you really need the tight feedback. It's a rare game that responds that quickly to the keyboard to begin with.
If you're really dedicated to low-latency keypresses, I'd suggest a PS/2 keyboard using laptop-style scissor switches, if any exist. Another option would be Cherry MX Blue keyswitches, which activate closer to the top of the stroke. There's some hysteresis, though, so if you're jamming on the same key in a game it's easier to miss a stroke.
I went crazy looking for a keyboard several years ago, and ended up a connoisseur. Definitely my most boring interest. :-)