I would always go for a card that gave about 300 - 400 images. As I normally fill a card up when I'm on holiday, I don't want to lose more than that should a card fail. I've spent so much to get those pictures, which may not be repeatable (are many pictures?) because they are in another part of the world.
The format, either Jpeg or RAW, greatly affects the numbers image that can be recorded, so it you switch from one to the other your capacity coulf be halve if going from Jpeg to RAW.
When I had the Nikon D70 I had 2GB CF cards. When I got the D200 I went to 4GB cards. I had all my cards stolen so got 8GB CF cards for the D300S, but also 4GB SD cards as I initially planned to shoot RAW to the CF and Jpeg to the SD, (the D300S can take both in case you didn't know

)which would have been a similar number of images on each. I changed my mind and now use the SD card as an overflow from the CF card.
I also fill cards when at sporting events with continuous shooting, which can fill up a card quickly, but again, I don't want to lose too many images should a card fail in probably the situation where it is most stressed.
I don't want to have just one huge card, again, if that goes wrong, what do I use?
When it comes to brands, I would go for a name brand. I had a MyMemory CF card a few years ago, which was quite fast (and cheap), but I would occasionally get the odd corrupted image when using continuous shooting.

So I've gone for Sandisk since, and have never had a corrupted image.
CF cards are more robust than SD cards, so again I would think of quality of manufacture. I was showing someone how to transfer images from their memory card a few weeks ago, and she had a very well worn no name SD card, and when I was pushing it back into the camera, the card disintegrated between my fingers.

That is the first time I've had that happen, but I've seen it can happen, so a good make and keep them in their little protective cases so they are not being crushed or worn when in your pockets or bags.
It is getting very hard to get smaller capacities, as the average capacity they make is larger, they make less of the smaller capacities. There is obviously nothing to stop you changing a card before it is full, which is something I try to do. I don't want to get to a point where I fill it up at the wrong time.
