Ok...so I'm late to this one...fuck off.
I just got done putting a new PC together primarily for video editing/processing (should work pretty much the same for multimedia use). Basic specs are AMD 6400 "Black Edition" dual core CPU, 4GB memory, 300GB HDD, and an Nvidia 8500GT 1GB video card all with an Aseus motherboard (no clue which one, though). It destroys movies like a bastard. My old PC would encode movies to DVD at roughly a 1:1 ratio...this one clocks in at over 5:1 usually. Takes about 20 minutes to encode a full movie. It's really just disgustingly fast. All in all cost me about $1000.
Observation: Why can't anyone make non-gay cases these days? I didn't want one with a wondiw or fancy lights and shit...but it seems like that's 99.9% of the market these days.