Speed will help with burst shooting, and with downloading times. I don't worry about it too much, though. I view cards as a commodity and buy based on price. I'll read online reviews to see if there are any systemic problems, and if not, then I'll buy.
IMHO, 1GB is too small for raw, and 2GB is pushing it. I'd look at 4GB as the sweet spot. I personally use 8GB, but some people argue against putting all your eggs in one basket.
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more! is a good source for memory cards, and they have good user reviews which help identify any systemic problems.