If you have a decent reader (ie. an external that cost > $10) then you'll notice a difference when copying to/from the computer. Not a huge deal, but it's nice to have decent transfer rates when you transfer 4 or 8gb.

I have had great luck with Transcend cards, and they're relativley cheap. I $15 for 4gb, $20 for 8gb, $35 for 16gb (wow!).