In reply to anomm:
From the Needlesports site:
"When starting out it is hard to know what sized cam to buy first, and whatever you do get, you can guarantee that it won't be the one you need when in extremis on your next lead! However, we would suggest a Friend 2 or equivalent as a first cam on the grounds that it covers a size of crack that is almost too small to hand jam and thus one where you are likely to want to get protection in with some urgency. If you are climbing mainly on grit/sandstone,granite then, having bought a size 2, we'd suggest working your way upwards in half sizes until you reach 3.5 and then working your way downwards from 1.5 to 0. If you are climbing maily on mountain crags/limestone then we'd suggest reversing this, ie buy 1.5 down to 0, then 2.5 up to 3.5."
This has done me well so far in getting my first few cams.