To keep it simple I would just get a decent 4 channel amp and use the front channels for the front speakers and bridge the two rear channels for a sub.
For a sub the Image Dynamics ID10V3 would be a nice SQ sub that doesn't require too much power. Or the Ascendant Audio Atlas is pretty cheap right now at 100 bucks.
http://www.cardomain.com/item/IDSID10V3D2
www.ascendantaudio.com
You can also pick up a Oz matrix Elite cheap
http://cgi.ebay.com/ws/eBayISAPI.dll...ayphotohosting
Very nice sub, You need a 2 ohm stable amp though.
What kind of budget do you have?