i take it I'd need an out put (so a graphics card) and then use the DVB card as the input,
The DVB card is where you plug in your regular sat, terrestrial or cable feed, just like you would with any set top box. Depending on which motherboard you go for, it will most likely have onboard graphics and output options like HDMI or DVI. If you haven't made the jump to LCD/Plasma yet, many motherboards also have s-video out as part of the onboard graphics. If not, you can put in a cheapo video card. If you want RGB/scart or component, that gets a little more tricky, but there are various custom cards for this purpose as well. I haven't looked at it too closely, as I didn't need one myself.
i am right in thinking that this will pick up all the cable channels that exist down the feed?
Yep, they will pick up anything a Dreambox will pick up.
so i take it i'll need a reader and then do the keys and that to the card?
There are two ways to go about it.
1. Hardcam. The CI slot on DVB cards is just for hardcams, meaning, you'd use either a subscription card or a dodgy card running decryption software. There are various discussions around here about Zeta, Joker, etc, that might make worthwhile reading.
The benefits are less CPU usage, faster channel changes, and direct compatibility with software that won't allow soft cams. The FloppyDTV card is popular because it has an onboard CI slot for this purpose, and is one of the few cards that can be made to work with Windows Media Center.
The negatives are that you have to invest in extra hardware, and that it seems to be slightly more of a headache to get it working.
2. Softcam.
A softcam works just like what you'd be familiar with from a Dreambox. No cards or hardware required. Popular choices are the Yankse/Fenrir combo and Vplug. The former is slightly outdated, but I think it still auto-updates for UK cable. I believe there are also a few other options.
The card I use comes with an optional CI reader, but works perfectly fine with a software emu. Without it, it only decodes FTA channels.
The benefits of a softcam are that it's just software, and very easy to set up.
The negatives are higher CPU usage and slower channel change.
If anybody thinks I've missed something obvious, feel free to chime in.