A bit of a challenge for you guys (HTPC Build)

So I'm creating an HTPC out of a build I already have. It's old, but I think it will be perfect for my needs. Basically I want to be able to play music, movies, and other downloaded content over my samsung 32-in HDTV.

Here's what I have from the old system:

ASUS A8n-SLi Premium (PCIe enabled, but will ATI cards work?)
AMD 3800+ X2 Dual Core Processor (2.0 GHZ stock)
2 GB Corsair XMS Ram

What I need:

-A cheap ATI Radeon card that will allow me to route audio and video through to my TV
-A case
-A PSU

My budget for this is about 200 bucks. I've seen radeon 4xxx series for around 50 bucks, but not sure what kind of PSU or case would work. I'd like something smaller but realize my motherboard is large and that may be impossible.

Thanks in advance!