The entry level DSLR from Nikon, D3000, supports a max framerate of 3 FPS in "Continuous mode".
Can you tether the cam to click photos directly into a PC at such a high speed? (Continuous mode?)
by Linus Kleen
Shooting tethered seems to work very similarly to shooting with an SD card: the camera stores pictures in its internal buffer before transferring them to PC, and therefore can shoot as fast as the buffer can handle. (At least, I haven't noticed any slowness.)
However, don't expect to be able to shoot RAW photos indefinitely at 3 frames per second: your USB connection just won't keep up. I just did a little test with Aperture's tethering, and found that I can sustain a speed of roughly 1 frame per second shooting RAW.