have you checked my HTPC starter guide yet?
i explain alot of things in there an i suggest builds too!
step#1 is really to decide the form factor you want...
size of a mac mini?
size of those old VHS tape player?
size of an AUdio Video receiver?
then once you have decided, its easier to select a build from my guide...
hope this helps and yes, for $300, i think u can do better by building your own,,,
but decide the form factor first, as this is crucial...