For the past couple of years I've been using an old 1.7ghz computer to do the tv-capping, with the main problem being that I was pushing it to its very limit in order to get 600x342 resolution. Now, with the new computer, I'm able to make things full size (720x576) and the results look a lot better.
I had to download 3rd party drivers for the card (Pinnacle PCTV) so that it would work with an x64 processor and have spent the afternoon chasing errors, but it all seems to be working well now. I'm sure you'll all notice the difference straight away, even though the filesize remains the same.