A friend of mine was doing the same thing as you so i gave him this link:
http://www.maximumpc.com/article/features/maximum_tv
I just started from scratch as well but i had my old case (ANTEC bigger one)
I have this and I do just fine with HD movies:
CPU: AMD Athlon 7750 Dual Processor 2.7Ghz ($50) (amd always gives you more bang for your buck
MOBO: GIGABYTE GA-MA78GM-US2H ($99) includes integrated HDMI output using ATI3200 on board , Blu-ray playback supported by high quality 106dB SNR ALC889A HD audio onboard. Something with HDMI output and a quality brand name video card is ideal.
If you don't find onboard video you can always just get a ATI HD video card on newegg. ~80$
RAM: DDR2 1200+ i believe got 4GB for $30~ Frys
Powersupply: i went with a silent 500w 29$
FANS: Default on the amd cpu, works fine and low heat and its always on for me.
Case Fans i have 3 in my case but that's because it fits, all 80MM I believe and all silent.
Storage: I have 640GB Western Digital Sata drive + a 500GB WD external drive.
Network: Although the mobo has integrated NIC for your online, if your making it a htpc go wirefree and get a wirless card ($16)
http://www.newegg.com/Product/Produc...82E16839121008
Drive: you can get a blueray internal drive + HD-DVD if you buy these discs in stores,
http://www.newegg.com/Product/Produc...82E16827136133 ($109)
Case: As we saw above those falcon drives depending on your preference and price range:
you can see more here:
http://www.newegg.com/Product/Produc...rde r=REVIEWS
I hope that answers all your questions
