Well as for sound, this is a completely seperate setup.
I simply use the 'on-board' sound of my shuttle PC, which I find is more than adequate.
There is a spdif optical (toslink) output at the back of my PC which I connect to my surround sound setup, which in my case is an 'Acoustic Energy Aego P5' system which was originally £600 but was reduced to £300 when I got it a couple of years ago.
I'm very happy with the sound quality of this system and it has tiny wall mounted speakers (the centre speaker can just be seen above my screen in this post:
Sapphire Screens
If I had the cash and with 'wife approval' I would get a full size speaker package and a good quality amp/processor, I'm not saying your setup needs improving as I know very little about your amp/speakers. I think the ones with auto calibration are a very good idea as no matter how much you spend on a speaker system the benefits will be completely lost if it isn't setup correctly
The trouble is you'll end up with a brilliant setup and will be left waiting for the high def media to appear like most people on these forums
Also, I was amazed at the difference in quality of DVD's, it's very frustrating to buy a brand new DVD only to find that it's a crappy transfer and there are loads of artefacts visible, which I find very distracting!
To show off what your setup can achieve stick to aimation films like shrek and the incredibles as these are fully digital and don't suffer from transfer problems like 'ordinary' films.
You may also have a similar problem as myself in that I need a 10mtr cable between my HTPC and my PJ which means I currently use VGA, which while not as good a connection as DVI (as it needs to be converted from digital to analogue and then back to digital by the PJ) means that I don'tsuffer any problems due to cable length.