To amp or not to amp?

I am currently running a 7.1 surround sound system in my home theater with a Denon AVR 3802.It has been brought to my attention that if I purchase an amp and use my Denon as a pre amp I will greatly improve the sound.I have a couple of questions,first do I need to get a amp that has 7 channel outputs or would just buying a 2 channel amp help me also?There is a big difference in price as you know and I am on a bugdet of about 800.00 dollars.Can anyone suggest what type of amp I should be looking for and what brand would be in this price range?I was looking at both 2 channel and 7 channel amps made by B&K but they are very pricey so I'm looking for an alternative. Thanks in advance