You can use this article
as a guide (and follow the included link for instructions on how to format the cards)
Note that the back will not take advantage of the added speed of UDMA cards as UDMA support was first introduced in the Aptus S model, so you might want to save a few shillings by not going with the latest and greatest
Personally I prefer smallish cards as they reduce the risk of loosing a lot of data in case of damage/ loss/ theft etc. My biggest card is 8GB...