I usually tend to use 4GB Class 4 cards mostly now... though at one point I bought a more expensive, high-speed 8GB card.
I've owned about a dozen and a half different SD cards over the last six or seven years, ranging from 256MB to 8GB, and I've never had one wear out and fail on me. That's pretty reassuring, but inevitably, it's going to happen that a card will suddenly cease to function when I go to import my photos. Thus, I'm not too keen on huge memory cards, really... I like the idea of using smaller cards and just swapping them out as I fill them during a shoot. In fact, I rarely need to use more than two during an outing. At 4GB, I can hold something like 250 photos per card (12MP, 12-bit, compressed RAW).
Fast cards are cool, but you pay such a premium for those extra Class points! Class 4 works just fine for me...