Most important difference is that the 520 is no longer available and the 820 is. Both are D series systems and you would be better off looking at the E ranges now in any cases as they are more up to date and better all round.
To anwser your specific question the 820 is a much more capable platform. To some extent you could configure an 820 and 520 to similar specs but even so the 820 was always a more robust and higher spec system. Some of the key differences between them were:
- The 820 supports slightly better
spec CPU's at the top end.
- The 820 can have an
nVidia Quadro video card, the 520
can only be configured with the on
board Intel GMA.
- The 820 can be
configured with a 1920x1200 res
screenm the 520 maxes out at
1400x1050.
- The 820 can have an
intgrated WWAN cellular adapter.
- The 820 has a modular device bay can
be used for both a secondary hard
disk or an additional battery, those
options are not available for the
520.
- The 820 has an Expresscard slot in
addition to the PC Card slot, the
520 only supports PC Card.
- The 820 has an onboard Gigabit Ethernet
capable adapter, the 520 only
supports 10\100Meg Ethernet.
Any one of 2,3,5 & 7 would be enough to make me choose the 820 over the 520 if I had to make a selection. I certainly would not buy a business laptop that only had a 10/100 Ethernet adapter and having an option for an integrated second battery\hdd is extremely useful.