PC Based DVR

I don't have a specific question, but would like to open a discussion of and get recommendations / thoughts / opinions (no lack of those) on building a PC based DVR. I am interested in building one as much for the challenge as for the use I would get out of it. Plus, I have enough spare parts to potentially build a PC DVR with minimal cost (I hope).

First, let me describe the environment. I have a Samsung 1080p 46" LCD TV that has inputs for HDMI (mulitple), component, coax and S-Video. I have a Samsung HT (non Blu Ray) connected to the TV via HDMI. I have a Comcast HD STB conected to the TV via DVI to HDMI cable. Internet connection is via AT&T DSL.

What I think I would like to do, my assumptions and potential issues.

(1) I would like to record TV programs from the Comcast STB for timeshifting. I assume the STB would have to be pretuned to the channel as I assume the DVR would not be able to switch channels on the STB - this may be incorrect as I think I have seen some devices that have remotes that will control the STB on a time/date schedule, but these devices maybe standalone devices rather than PCI cards with remotes. I am not interested in recording from OTA. The output from the STB to the DVR could be coax, S-Video or component (preferred). Therefore, I would need some sort of PCI card to receive the output, coupled with software to activate recording at the appropriate date / time.

(2) I would like the DVR to have the capability to receive Netflix over a WiFi connection to my home network and stream this to the TV. The network is currently 802.g.

(3) I am open to purchasing a Blu-Ray DVD drive for the DVR, but I can initially go with a spare non-Blu Ray DVD drive I have.

(4) I would like the ability to play some of my online games, especially WOW.

Objectives 2, 3 & 4 are relatively easy to address, with existing hardware. I have spare processor, MB, HD's, power suppy, video card, etc to assemble a system. The only things I am missing are a HTPC case, a spare Wifi adaptor and the PCI Card / Software for recording.

Objective 1 is the tough nut to crack as you need the right card and software to take receive and time the recordings. Beyond finding this particular card / sfw, I have a spare P4 2.8 Ghz processor and MB, which presumably would have enough power (maybe marginal) to do the recording. The MB maybe an issue as it is not PCI-e capable and the needed cards maybe all PCI-E.

I know a system can be built to do what I want, but I want to do this as cheaply as possible using as much of the spare parts I have on hand. Budget wise, I could go upto $300 to $400, but that is stretching it. I would welcome yours thoughts, suggestions, experience and opinions on this.