I have an HTPC in a Silverstone case, use a very low noise Seasonic 330W power supply, it only has one 80 mm case fan and in a years time I've never had heat issues. The machine has 2 hard drives, 2 dvd burners, 2 gigs of ddr ram, sound card, and the 330 keeps up admirably. I was gonna get a 2400XT as well and I'm gonna upgrade my PSU to a seasonic 380 at the same time.
I've been doing some research to build a new HTPC either at the end of this year or the beginning of next year.
If you plan on watching BluRay or HD-DVD then you should buy the E6300 instead of the E2140. Also I would get a nVidia 8600GT because it offloads video decoding from the CPU to the GPU. So far there has been no reviews of the 2400XT as to how well it can decode HD video content. See below for 8600GT video decoding benchmarks:
Als o, nVidia cards traditionally draws less power than their ATI counterparts over the past 3 generations of GPUs. That means less heat is emitted by the GPU.
Seasonic S12 power supplies are really quiet premium PSUs. I have the S12 500 and I cannot hear it over the ambient noise. The S12 430 is the quietest PSU out of the entire series, but the S12 330 or S12 380 will have no problems powering up your proposed system.
The Lian Li PC-V800B is the very same case I am considering for my next build. It is small and has good ventilation, but large enough to fit a standard ATX motherboard. One problem is that the PSU is at the front of the case, so a quiet PSU like the Seasonic S12 is a must.