I want to buy a new SD-Card for my Canon EOS 1000D. Is it worthwhile to buy a high speed card? What is the speed limit of the controller in the camera?
Rob Galbraith did an experiment with a Canon 450D here which may be useful to you: http://www.robgalbraith.com/bins/camera_multi_page.asp?cid=6007-9424 He seems to suggest the camera will write at max of 11-14MB's. The 450D has a similar sized sensor (similar size photo) and the same image processor so your 1000D will be similar.
If you shoot in JPG a high speed card should not be necessary (the 1000D will shoot JPG continuously while writing to the card and never filling its internal buffer). But while shooting in RAW the cameras internal buffer will eventually fill up, a faster card will allow you to get back to shooting sooner.
I did quite a bit of research and the consensus seems to be that a Class 6 card will be worthwhile (faster than a Class 4) but that going with a card that's faster than a Class 6 doesn't provide any meaningful speed upgrade.