Robert,
The comments other have made are on target - you definitely want a fast processor and quiet fans. The All-in-Wonder card uses software codecs for video, so it will put more workload on your CPU than a Hauppauge card will.
You have a few options for software Microsoft, Snapstream, and Meedio are the best options if you can wait a few weeks. Snapstream has BeyondTV, which is the PVR component and is releasing BeyondMedia shortly - which is the audio and add-ons side. Meedio has the audio and add-ons available now, and is adding the TV/PVR features this month. Both Snapstream and Meedio offer 15 day trial editions - try'em out and see what you like best.
You can and should get a remote control for your system - the ATI Remote Wonder, FireFly and iMon are good options.
All the video software I mentioned can control your tuner box via serial or IR. Your HTPC can share the tuner with your Tivo, or you can have the HTPC connected serially through the Tivo and use the IR to change the Tivo channels. All depends on what you want to do with each component - and how likely it is that someone will change the channel while the HTPC is recording your favorite show
I put in a second DirecTV tuner for my HTPC, and have a Sony Tivo/Directv receiver as well - that way, there's no conflict over tuning.
Good luck with your projects!