noooo, stay away from ati when it comes to htpc. i`m not being an intel fanboy but really, if you want a good-good htpc its either atom or the celeron e3300, google it, its about as fast as a c2d and it only uses 20watts.
for the video card, hd5550(hd5450 will be grinding hard so get some headroom) or gt430, i lean on nvidia because they pass hdmi audio without connectors and they have the future 3d output.
70$(ouch) termaltake lanbox(aint it cute?) http://www.newegg.com/Product/Produc...82E16811133045
50$ celeron ftw(its a god**** c2d for crist sake, they just took out 2mb of cache) http://www.newegg.com/Product/Produc...-264-_-Product
reviews : http://www.xbitlabs.com/articles/cpu...0_8.html#sect0
(ati seems to eat a buttload of power compared to intel, thats why i lean to it)
60$ motherboard(you can pick anything but i preffer those mobos that has ddr3, less power consumption and more headroom for OC, well, if you plan to) http://www.newegg.com/Product/Produc...82E16813131650
(you have an option between ddr2 and ddr3 on this mobo.)
50-70$ GPU(its either these two and dont go below, because you`d suffer when playing high-res and with those eye-candy enchancement, it doesnt hurt to give a headroom but it freakin hurts when you put overkill parts on it. ex: gtx570) http://www.newegg.com/Product/Produc...-397-_-Product
46$ ram (2x2gb is more than enough) http://www.newegg.com/Product/Produc...82E16820104206
sums up to around 300$, tho lacks the blue-ray room and a couple of harddisks, if you change the case then you get it more cheaper.