CPU
Surely though, if you have a powerful enough CPU, you wont have to worry about your motherboard or video card having to assist in the decoding process?
My own personal thoughts are that with HD material, getting video cards to assist in decoding is a bit of a minefield right now (there are huge threads in Avforums and avsforums regarding video cards and accelerating different codecs), so its best to make sure you cpu can do it all on its own.
Personally, I would opt for an Intel C2D as it will be able to do all the decoding on its own.
The last thing you want is to go through the trouble of putting together your PC only to find you are getting skipped frames when you view 1080p material.
Video Card
If you use an ATI card stay away from the ATI 2400Pro. 2400XT and up is fine. The Pro version has problems with some codecs.
Also, someone stated that the NVidia chipset assists in the decode of most common codecs. The codecs that would need assisting with will be with HD material. Im pretty sure that NVidia will struggle to assist with x264, H264, etc. I could be wrong, but a link would be nice.
Memory
I would stick with 1GB-2GB ram, as others have stated.
Hard disk
400GB is ok, but remember go for something as big as you can afford. I would go 500GB as the bare minimum. The price increase is only slight. Think, if in the future, you rip a Blu-ray or HD-DVD disk onto your hard disk and 30GB gets used up by a single movie? If you decide to put in a DVB tuner in the PC and record programs on your hard disk then think how quickly the space will get used up? One thing that a media PC needs is hard disk space.