Think thats about it.
You could get a combi drive like the LG BR / HDDVD one (GGC-H20L is a BD / HDDVD reader, GGW-H20L is a HDDVD reader and BD reader & writer) as that covers both bases (but Toshiba has just pulled out of HDDVD so the format is now expected to die out, the LG ist still cheaper than most BD only drives though and you can buy all the cheap HDDVD films when they go clearence).
If you go for a graphics card that supports HDCP and does full offloading of BD / HDDVD then you can get away with a low speed and cool running CPU. I have an old Athlon AM2 4200 chip and an ATi HD3470 graphics card, this has no problems playing back 1080P blueray discs with only 15% cpu usage. Without the HD3470 I can only play back at 720P and sometimes it stutters.
Look for drives that bundle PowerDVD with them (the LG has a basic version that outputs to 2 channel or SPDIF interface, good enough for a basic system).
Most flat pannels now come with built in HDCP and all modern graphics cards have it built in as well so that should not be an issue. There is currently a problem with AnyDVD playing back some BlueRay discs that are protected by a feature called BR+ (or its BD+, cant remember). If PowerDVD finds a BR+ enabled disc that is not AACS encrypted (AnyDVD removes the encryption) then playback fails. Easy fix is to just disable AnyDVD if you come across the problem until its fixed (as they are bound to do).
If you are building a PC from scratch purely for HTPC use then I would hold off fora short while. The BR / HDDVD fiaco will have finally sorted itself out and new motherboards will be along that support integrated graphics powerfull enough to playback HDDVD and BD without using a seperate graphics card. Look out for ATi chipsets from the 790 series and nVidia from the 8000 series, both are expected by the end of March, then you will just need HDD, optical drive, some RAM and a case to make a full system
Hope that helps