Agree with byfordr if the oem is cheaper go for that if not then get the retail version. As he says you can download the software so the only advantage the retail has is the ide cable and you get one of them with your motherboard anyway, you'll get enough screws etc with your new case to connect up the hard drive. It is worth checking on the guarantee though. I've never really worked this one out but its true in a lot of cases, if you buy the dearer retail version you get a 1 year guarantee, but if you buy the cheaper oem version you sometimes get a 3 year guarantee. Doesn't make much sence to me but there it is. And its worth spending a little bit more now and getting a bigger hard drive especially as we're only talking £10 extra.
Go with the 80GB retail boxed Western Digital you suggested. It will have all the RIGHT threaded & length screws you require. The correct 80 wire ribbon cable and detailed fitting instructions. As this is your first DIY PC project go retail box. It will save you the hassle of trying to find the correct screws etc.,