MythTV is a terrific program, but as of now, and maybe for quite some time, there are no software players for Blu-Ray in Linux. So if you are intent on BD on your HTPC you will need to run Windows in some form or other.
If you are going to run an Nvidia card, you do not need to worry about having a native HDMI port, since the card is not, nor can it ever be, capable of HD-Audio from Blu-Ray. You will need to use a Xonar sound card for that with an Nvidia graphics card. Otherwise your only option is an ATI card, which will pass HD-Audio through it's HDMI port. Or through it's DVI port using the DVI-HDMI adapter.