vehelpfulguy is so right again (this bloke gets everywhere dont he)
We have just had sky+ installed that has an 80 gig hard drive and think that is enough for around 40 hours and it is truely amazing it is so simple to use even my dog can proggrame it (well nearly but he is working on it) and you can even send a text from your mobile to it to record a proggrame as well as proggrame it on line.
If you already have sky it is well worth upgrading to sky+ and depending on what package you have from them you might even get the box for free or at the worst it is �10 a month to hire it.
Another route here is to buy a sky+ box on e bay I have seen them for well under �100 and they will work with your subscription with sky but you will have to phone sky up to tell them to activate your new box,also have seen new 80 gig recorders on e bay going for around �90, if you just want to buy one of them (amazing isnt it when dvd recorders first came out and it was not that many years ago they were around a grand)
as for being portable dont think there is anything on the market at the moment that is apart of course from the i pod and other mp4 players etc, but the sky+ box has a usb port in the back of it but have gone into this on here to see if there is a way to upload videos from the box to a portable hard drive and have been told that the usb port serves no reason at the moment so hopefully sky will make this happen soon