Current media system is a dual core amd, 2gb ddr2 @800 and a 9500gt 512mb card gddr2 w/hdmi out, 500w psu, total price (already had spare hdd/optical drive and case) was a little under $300 (including am3 mobo w/PCI-E 2.0 x16). networked to file server.
video card was kind of overkill for just playing vids, but lowest priced cards were around $30, so $20 more wasn't that big of a difference. Most complete systems around that price were integrated graphics(6000 series), single core, and intel atoms.
Adding a case, hdd, optical drive...though i'd discount hdd and optical, both are reusable

would only bring it up to around $400, with still more capability then oem.