Hello Katie,
Like you, I recently came to the conclusion that the hard disc on my Sky+ Box was on the blink following an extensive research into the ever useful AV Forums and others. And like you I was also left pondering what disk speed to go for (I have the same Pace box as you).
To cut a long story short, I followed the advice from Zappo contained in the following thread:
Sky+ HDD Upgrading, and a couple of other things!
Zappo pretty much summed up the key conclusions of everything else I came across (thanks Zappo!

) and I went for the most cost effective option of the Seagate drive he links to from Ebuyer. This arrived today, was installed within about half an hour and touch wood everything appears to be working fine. Obviously, I won't be able to tell if the freezing problems occur again until I have recorded a few programmes (I found that the more space being used on my old drive the worse the problem).
Be warned! My experience of Ebuyer is quite negative in terms of misinformation and let down in delivery. On the three occasions that I have used them they have stuffed up my order in some way or bolloxed the delivery (the aforementioned drive should have arrived last Thursday). But they eventually sort things out, are cheap and I did squeeze some compensation out of them for inconvenience ...
Don't forget that you will need the right tools if you are installing the drive yourself. If you are unsure of what to do, I used the following link for instructions:
http://www.morlocks.co.uk/disc-change-v2.htm
I'll try and keep you posted if I experience any problems: I have stuffed the planner with stuff to record tonight and am keeping my fingers crossed!
Cheers,
Ricky