Feeling sorry for you, here's my take:
The 25 series has EX capability. The 20 series does not (neither does the 8000, but it is capable of Logic 7 on multichannel sources while both the 20 and 25 series are only able to do it with 2 channel sources).
The main other difference between the 20 and 25 series is 6 amps in the 25's and only 4 in the 20's. The 20 series will decode into 7.1 but you need a separate amp to drive the channels. In the 520, you can reassign the amps to power whatever you want, including a separate preamp. In other words, you could get a different power amp for the front channels, then use the 520's front channel amps to power the additional 2 rear channels, or use the power for a component in another room, etc. I'm not sure if the 320 will do this. HOWEVER...The 25 series 2 extra rear amps can be used to power components in another room and give you multi-room capability without buying another power source.
There is more power in the 500 series than in the 300 series. " 325 is 7x50 watts 525 7x70 watts.
The 525 has Digital Bass Management of 6-/8-Channel Direct Inputs
There are, of course, other small differences, but the main things are:
A) Power (more for the 500 series and more amps for the 25 series)
B) EX decoding (yes for the 25 series, no for the 20 series)
C) Ability to assign the amps to external sources (yes for the 20 series, and no for the 25 series).
My recommendation: Get the 320 (? at jandrworld.com). It's more than enough for a second system. If you want EX decoding, go with the 325.
I hope this makes sense. It's 5:30 AM and I've been up all night after 5 hours of sleep.