The XSi will accept this card and will be able to take advantage of its speed. Any point and shoot camera that accepts sdhc cards will accept this one. As mentioned earlier, the higher the class (i.e. Class 10 vs. Class 4), the higher the speed. However, most point and shoot cameras will not benefit in any way from the extra speed, so you may be better off buying a lower class, cheaper card.
At $13 though, it's probably worth it to buy this one if you ever plan on doing any kind of video shooting or high speed photography.
I have a Canon Vixia HG20 camcorder and have used a Tangent SDHC class 6 memory card which worked fine. I decided to upgrade to a class 10 this time, but when I put it in the camcorder it says that MXP OR FXP cannot be recorded on it. MXP & FXP have the highest HD resolution. I changed to this higher class because it said it was compatible. Does anyone know what is wrong or what I've done wrong, before I return it to Amazon?
Ivan, from what I've read, the only thing you're likely to have done "wrong" is bought Transcend's Class 10 instead of Sandisk's or (apparently) Tangent's class 6 or better SDHC cards. You could first try formatting the card in your camera, or first formatting it with the special tool [available free at https://www.sdcard.org/consumers/formatter_3/] then formatting it in your camera, but don't hold your breath.
I know the higher # of class means higher speed. I have a similar question too. This must a newer memory which I could not find in from my camera's manual. Is this SD card compatible with Canon XSi camera?