I have been researching the same topic for a while now, and based on what I have found so far I can assure you that till date there is NO PC based solution that even comes close to the functionaluty that Tivo provides. I have been a Tivo user for over two years now, and have recently started playing around with the idea of adding PVR capability to my HTPC. However, I am doing this as an ADDITIONAL PVR solution ... not a replacement for Tivo.
So in case you are planning to abandon your DirecTivo in favor of an HTPC based solution only, I would strongly advise you to run them parallel for a while before finally going one way or the other.
With that said, let me actually try to help you now
1. I have BOTH the AIW Radeon 7500 and the LeadTek TV2000XP installed on my machine and I have done a lot of comparisons between the two using my cable rececption. Based on what I have found, the TV reception quality between these two boards is almost identical. Using default settings, the picture from ATI seems to be just slightly sharper than the picture from LeadTek ... but definitely not something you'd notice unless you run the two side by side.
However, this is just one aspect of the comparison. TV reception is one factor, but that is not all. With the AIW you get the excellent wireless RF remote for free. I haven't used the LeadTek remote that much, so I can't comment one way or the other on that, but it is a regular IR based remote.
LeadTek WinFast PVR uses TitanTV to get TV listing. ATI uses the GemStar Guide Plus system. Guide Plus is severely limited in functionality as reported by many reports. I don't know how good or bad TitanTV is in this respect.
LeadTek costs almost one third than that of the cheapest decent AIW that you can find ... so that might be something to consider.
2. Unless you are coding up your own solution, your choice of the PVR software will make this decision for you. Most PVR softwares use TitanTV to get the guide data. This list includes ShowShifter, WinFast PVR, PowerVCR II etc. SnapStream PVS is a notable exception. They used to use the free TitanTV listings; but from April 03 onwards they are going to provide their own monthly payment based ($4.95) TV guide service.
3. It is hard to say ... but both ShowShifter and SnapStream PVS looks interesting. Cyberlink PowerVCR II might be good as well. I would be eager to get feedback from someone who has actually used one of these products ... especially ShowShifter.