I was going to buy a new memory card for my Canon 500D and I was wondering if SanDisk Extreme III SDHC Card 8GB which has a write speed of 30MB/s would be a good buy. How can I find out if my camera supports 8GB cards and can make use of 30MB/s speed? The price difference is not really that much.
The Canon 500D takes SDHC cards up to 32gb. Any Class 6 or above will make sure that there is never a slow down from the cameras buffer filling up, and the 30MB/s card you mention is a Class 10 card so plenty fast. I just bought two of those myself to replace my 16gb PNY as I get a bit of slow down shooting raw occasionally
The canon 500d supports a maximum of 32GB and any class or grade cards work fine as long as they are above class 5 for video rcording.