Depends on what camera you have and what the write speed is. Obviously you want a card thats fast enough for your camera to write to to minimise any possible buffer filling.
Then there's getting the pics on and onto your PC.
Rob Galbraith usually has some decent speed tests on various makes, i.e. here's the one for the 50D
http://www.robgalbraith.com/bins/camera_multi_page.asp?cid=6007-9672
Personally I stick to Sandisk as a brand now as I've never had one fail, unlike some other cheaper brands.
just had a look at them and the reviews say they are a bit slow?? any thoughts and people are saying try the Sandisk Extreme III CompactFlash (CF) Memory Card - 4GB - 30 Mb/s Version for an extra few quid would this one be better? thanks
one on order now thanks all for your help. just need a card reader now!!