Quite a few folk use class 10 cards without any issues to my knowledge and in some cases, such as running Daphne (laserdisc emu) they notice an improvement in the smoothness of playing speed. So its probably fine.
When you talk about the speed of reading and writing small files, it's not the class that you're interested in. The class only relates to the continued write throughput/speed on large write operations. What you need to concern yourself with is the I/O operations measure, which unfortunately, is something that it's hard to get good data on.
Toms hardware have done a couple of SD card labs with the benchmarks that you need. Though if you've got anything less than something like a Sandisk Extreme, it's likely that the I/O measure on your card is likely to be relatively bad. Samsung's SD cards are good middle grounds in terms of price/performance on the I/O side, I run my OS off them.
My only problem is that when using Firefox or Midori, the file access light comes on and makes the browsing experience very slow of freeze for a few seconds. I am talking about the leftmost green light. What can I do to lessen this?