As far as I'm aware of, there are no commercially available external soundcards for a laptop that can process and bitstream HD audio codecs.
Going back to your question on LPCM, you can still get DTS-HD and TrueHD that way. The difference is where the signal is decoded. If you choose LPCM, the laptop decodes the audio signal and will send the uncompressed digital stream to the receiver.
Bitstream is often compared to a zip file. It is a compressed version of the digital signal sent out and the receiver will decode the audio signal instead of the computer.
Overall, the information IS identical whether you let the computer or receiver do the work. The difference between the two is related to how the receiver handles the signal. Some do not have the ability to apply bass management and room correction to LPCM, but can to Bitstream or vice versa.
From what I read on www.notebookcheck.net
, only the AMD Radeon 5600 series or higher can bitstream in a laptop, but I couldn't find any confirmation where someone actually did this.
If your laptop can support 7.1 channel LPCM via HDMI, then you should be just fine.