My MythTV Hardware
If you want to know whether your spare parts will work in a MythTV PVR, or what your choices are for the stuff you don't have, take some time to look at the MythTV hardware sectionon the MythTV Wiki.
Like many others, I chose to experiment with what I had first. If I were successful, then I would try my hand at building a Front-End only system as a replacement STB for my two HD televisions (Sony SXRD and LG Plasma) and leave this experiment in place for the Back-End. But first, lets discuss my combo Back-End/Front-End system.
My "Combo" System
I happened to have an "older" Intel Pentium 4 - 2.4Ghz system gathering dust that has 1GB of RAM, an 80GB hard drive, a DVD-ROM, a diskette drive and a few USB-2 ports. I chose to upgrade the AGP video card from an ATI Radeon 7000 to a EVGA e-GeForce 6200 for two reasons; to get a DVI output and S-Video output, and because I read a lot of stuff on-line that favored NVidia chip sets. 
That cost me $47.49 before tax, but also came with a rebate for $10. I also bought a low-end PCI audio card for the SPDIF output. In retrospect, that was $15 I could have saved since this system will really end up as a Back-End only and I can use the stereo output for the short term. So I spent a little under $54 for my system (after rebate).Since I want to watch off the air (OTA) programs and also some Cable TV programs, MythTV needs two capture devices. I looked into many different brands of cards. I needed one that had an ATSC tuner to receive OTA broadcasts via an antenna, and one that had a "QAM" tuner to receive the Cable TV digital channels. Now technically, the FCC mandates that local OTA channels (SD or HD) be sent over the cable TV system un-encrypted so I could have avoided the OTA tuner/capture card. But don't forget that the cable operator is not obligated to pass the HD signals untouched. They may (and likely will) compress it to fit their needs, and my cable provider does compress the original video to a lower bit rate.
I chose to purchase an HDHomeRunas my tuner/capture device (little white box sitting on top of the PC in the front view above). It has two tuners that are ATSC and QAM capable, and a 100Mbps network connection. It was about $30 more expensive than two individual capture cards, but I thought it would be easier to setup. It cost me $169 at Fry's. I probably could have got it cheaper on-line, but I am an impatient person. (How I am finding time to write this is amazing in itself).
By the way, if you are thinking that two tuners is not necessary, that would be true if you are recording only one program at a time, and your TV has its own ATSC and/or QAM tuners to watch something else while you are recording (or you still have a STB from your service provider). I haven't talked about the "operational challenges" yet, nor the cabling and antenna issues, but we will get to that. For now, let's simply say that I have an OTA antenna connected to one input on the HDHomeRun and a cable TV drop connected to the other input.
So there we have it. My combo PVR device with the external capture cards cost me $223 or so. So far I have a 5+ month payback to absorb, once I actually turn in the two cable TV boxes.
My TV Guide
TV listings are not free. Broadcasters can embed in their signal the Program Specific Information (PSIP) for up to 12 hours or so, but few do more than the current program or maybe the upcoming program. I have subscribed to Schedules Direct for $20 a year to provide me with the TV Listings compiled by Tribune Media Services. You can specify multiple lineups in your account, and I have created one for my cable service and one for the off-air antenna. Set this up before you begin configuring your system.
Now the ultimate challenge - preparing to install MythTV and the operating system.