"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."
