Logically, uncompressed NTSC (no audio) is about 29.7mb per second of video (720x480x3x29.97), USB 2.0 is capable of 57mb/s speeds, there's no mention of any hardware encoding on that TerraTec Grabster page or on the Articles page where the hardware encoders are pointed out, so there's a possibility it does send uncompressed video to the PC and the mpeg is software encoded on-the-fly with a fast compressor.

The technical specs say the faster the PC the higher the resolution of encoding: Technical Data - Grabster AV 250
(looks like XP only, damn, my video PC is W2k)

This unit might be work looking further into.