785 chipset mobo, Athlon II X2 CPU, 2x2GB DDR2, and you're all set. That chipset has good enough built in graphics to run 1080p no problem. Someone else will have to chime in on the tuner card, though.
edit: $120 CPU/mobo combo,
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.273663
Add
$40 $60 worth of DDR2, maybe
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145184 (hmm, DDR2 prices went up when I wasn't looking. Sorry.)
$68 gets you a Bluray drive, and it includes software that supports 2.0 channel audio. I think it's like $90 for the full surround version
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106326
I use 2.0 channel audio, so I went with that model. If you use surround, go for
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106292 instead at $100 which includes non-crippled software.
This little fella is a perfect PSU, gives you room to grow and dead silent. Also only $30 after rebate.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139008
Total uhh $310 after rebate, before OS. Could always throw in a 4770 for the leftover $100 or so you'll have after buying Windows 7, to add decent gaming power.
edit 2: Vista Home Premium 64 bit with a W7 upgrade coupon, $110
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116677
Silverstone HTPC case, $90 after rebate,
http://www.newegg.com/Product/Product.aspx?Item=N82E16811163117
I'm assuming he already has the case since it was mentioned, so will instead suggest this WD Green 1TB drive for $85,
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136317