Tomtom writes different software for different hardware versions.
The 520 and 530 are mostly the same with the 530 having newer software.
Similarly the 540 and 550 are mostly the same with the 550 having newer software.
- The 540/550 hardware has a built in sim card for LIVE services, while the 520/530 doesn't.
- The 520/530 have an FM transmitter, where the 540/550 don't.
- The expansion slot on a 520/530 is SDHC. The expansion slot on a 540/550 is microSDHC
- The 520/530 have a hanger style passive mount. The power plugs directly into the base of the Tomtom
- The 540/550 have an active mount. The power plugs into the mount, and the Tomtom docks into the mount
I may have forgot some other hardware differences, if there's anything in particular you're interested in just ask.