I would like to record from s-video and dvb onto the hard disc and archive onto dvd with good quality. Do I need a separate dvb card (Nebula?) and decent graphics card with analog i/ps or is there one device for all?
While you will get very good quality from a card like the nebula for digital terrestrial (as it records the stream as broadcast, so a later playback is identical to the original), for sources where you have to use analogue input to the PC you cannot get as good quality as this. Composite video is pretty hopeless. S-video is better, but depends as well on the capture card. The best is to use RGB input if you can get a card that will handle it.
Lots of people are using the Sweetspot card to do this and seem to like it a lot.
I don't know about that particular card, but the Xcapture and tv-100 cards seem to be the budget cards of choice - these both handle PAL/NTSC, 50 and 60Hz.
I moved from a Pinnacle PCTV Rave to the Xcapture (before SweetSopt arrived) and the difference was pretty significant.