If you haven't run into the HT primer/FAQ you should read that, some of this is covered there.
Basically, pre-outs let you use outboard amplifiers instead of the amplifiers in the receiver. The 220 is a regular 5.1 receiver, with 5 power amps built in. In fact, it does not have pre-outs at all. IT does have 5.1 pre-ins, though, which you likely would only use for SACD/DVD-A if you go that route. The 220 does not have EX/ES decoding, and if you dont plan on going the 7.1 route than this is not important. Bottom lime, the 200 is a solid 5.1 receiver, but you cant use outboard amps with it, nor any 6.1/7.1 stuff.
The 325 has 7 amps built in, which can be configured either for 7.1 setup, or a 5.1 setup, plus a second zone. Two different sources can be listened to in these zones. It sounds like this is what you are trying to do, ans this receiver is perfect for that.
It also has a full complement of pre ins/outs for 7.1 if you need/want that.