Where to start?

There is no substitute for listening to speakers when it comes to choosing... Most people just take the installers advice, but if you care about sound quality, you'll have to do some foot work. I assume you mean the Pioneer PRO-1120HD plasma display. You will need a special proprietary cable from the Pioneer 'media center' to the display. It comes with a ten footer... Not familiar with the 3801, but as long as it's not a total piece of junk you should keep it and invest in good speakers... you can always upgrade the receiver later, but you only get one shot at the in-walls...
It's usually easier to use a separate two-channel receiver to run the music in the rest of the house.

Oh, and it's DVI, not DCI...;)

Good luck,

Norm