As mentioned above, class 10 is really only important for HD video, and it also helps if you're doing large, rapid bursts. For normal photography, class 4 or class 6 will be fine.
I do find that there is a significant difference among brands, though. Sandisk is very good. They're a little more expensive than other brands, but they've also been among the most reliable. Lexar can be hit or miss: I bought a 32GB class 10 that gave all sorts of bad write errors, but was replaced under warranty, and the replacement is great. (The parent company to Lexar makes excellent computer memory, and the replacement experience left me positively disposed toward them.) A lot of people seem to like Transcend, but I know very little about them other than that they're much less expensive than Sandisk.
Personally, I've got a 4GB class 4 Sandisk and a 32GB class 10 Lexar (purchased on sale). The Sandisk just plain works, but burst mode is a little slow. As mentioned above, I had to do a warranty replacement on the Lexar, but the new card is great and there's a noticeable improvement in burst mode. Personally, I'd say go with Sandisk at class 6+, and I'd suggest getting more cards at lower capacity (less lost data if the card goes bad.)
In terms of money, right now it looks like 16GB is the "sweet spot," i.e. 16GB is generally less expensive than 2x8GB, but 2x16 is (slightly) less than 32. Factoring in sales, 16 and 32 are pretty close in terms of price per gig.