This is a copy/paste from another thread with a few additions,so please forgive the cross post to help others build a nice, affordable Boxee + HTPC. This is after a great deal of reading through sometimes hundreds of product reviews for each component...
Power consumption....go with a 45watt CPU like the AMD 4850e (check out the 5050e also) You may have this HTPC on 24/7 for torrents/TV recording so a power hungry CPU and motherboard design will cost you more in the long run. If you're not going to do video processing, heavy photo editing or high end gaming the combo below will serve you well.
Cost ....AMD has lower total cost, been rock solid
Motherboard...
Gigabyte GA-MA78GM-2SHP below is ready for Blu-Ray, 7.1 Audio Optical out, HDMI, gigabit ethernet, nice expandability, best in class, cheap. PS: It does support the newest 140W Phenoms for some superjuice down the road if you re-purpose. The
AMD 780 chipset on a mini ATX MB is a must fro HTPC and integrated Blu-Ray playback, thus no need to drop $$ on a discrete graphics card.
Flexibility....Blu-Ray drive, got most common sold for support and info
Drive Space....cheap, get 500Gb or better, you might keep an eye on quiet HD tech. specs.; if you record TV or torrents you'll want a beefy local HD
Case...get decent power output 450 watts+ for now and future; you can always pay more for a
high efficiency 80 PLUS PS
O/S...I went Win Vista Ultimate for MCE to be my DVR (trying to figure out how to get Boxee to recognize the TV shows natively or with some minor filename morphing) There are many fans of MythTV and other DVR solutions, so take a look at them.
Here's my under $500 HTPC with Blu-Ray and HD Audio (look for the Realtek ALC889 audio chip for solid sound for the $) not listed below is a scavenged 500Gb drive, TV tuner card(your call) and 2Gb Memory(DDR2800 would serve you well, MB below supports 1066 depending on CPU tho, read the specs on Gigabyte site)...all purchased from Newegg.com
HT CASE APEVIA(ASP)|X-MASTER-BK/500 $74.99
MB GIGABYTE GA-MA78GM-S2HP 780G RTL $79.99
CPU AMD|A64 X2 4850E 2.5G AM2 R $53.99
BD/HD-ROM COMBO LG|GGC-H20L SATA RT $119.99
Payment Summary: Subtotal:$326.45 Tax:$25.07 S&H:$31.70
Total Amount:$383.22
Conclusion and Results: Blu-ray discs play great, Boxee solid so far, can access my network shares, internet browsing on TV (a 42" 720p philips via hdmi)
Hope this helps the Boxee Windows DIY crowd a little.