I wouldn't go as large as 16GB for memory cars unless I was doing a lot of video.
I'd always say about 300-500 pics per card, as you don't want to be in a position of losing too much should bad things happen, the card corrupting or being lost. :shrug: Plus you have spares as well.
After having problems with MyMemory's own brand card 5-6 years ago I've tried to stick with Sandisk ever since and have never had a problem with either their CF or SD cards.
I've had no trouble buying from
7dayshop or
MyMemory, and they are usually close to, if not the, cheapest price.
Fast CF cards (30Mb/s +) would be the best choice for sustained performance imho. The fastest SDHC cards are 35Mb/s,(and a stupid amount of money btw

) whereas the fastest CF cards are 90Mb/s.
I think the D300S tops out at about 30Mb/s in transfering data from the camera to the memory card. I chose Sandisk Extreme CF (60Mb/s) and Sandisk Extreme SDHC cards (30Mb/s).