HTPC - How can I upgrade this one?

[kingofgrills]

I have a Gateway GM5420 Media Center Desktop PC, but I've never done anything to use the media center aspects of it, other than serve files from it to my PS3. Here's a link to the Gateway model, where you can view the stock specs, available slots, and different views.

http://support.gateway.com/s/PC/R/10...014159nv.shtml

I just switched from Dish Network to Comcast HD cable, so I'm interested in utilizing this PC as more of a media center now that I'm no longer on satellite. My only upgrades so far were to ditch the integrated graphics card for an NVIDIA 7600GS with 512MB DDR2 SDRAM and also add an additional 400 MB hard drive. It has a built-in TV tuner, with CATV VHF/UHF tuners, but the only outputs available are S-video and composite.

What I'd really like to do, if it's possible, is to upgrade and use the PC as a DVR, recording TV off Comcast and then serving it to other media hubs in the house, including my laptop and PS3. Is this at all possible, and if so, what will I need to change on my PC to pull it off?

I greatly appreciate your help with this one.

Thanks!