I don't know if anyone has excplicitly said it, but 2007 is the year that compact digital cameras got good at low light shooting. So far, all of the cameras introduced this year that I'ev tried have had great high ISO image quality. They're still not even close to digital SLR quality. But unlike previous compact digital cameras, this year's cameras are actually useable at ISO 800 or even 1600. So, I think you're in luck. And I think both of the cameras you mentioned will work well. I'd go with the Canon over the Fuji because of the manual exposure controls. They will give you more options if you're shooting action. But the Fuji is well-known for being a good low-light camera.

One useful feature that neither of the cameras you mentioned has is image stabilization. I think it's one of the most valuable compact digital camera features available right now - especially if you plan to do a lot of low light shooting. It won't help you freeze action. But it makes a huge difference for camera shake in low light. If you like the Canon PowerShot A640, I would recommend taking a look at the PowerShot A710. It is a lower resolution camera (7-megapixel instead of 10) but I think that's more than a fair trade for image stabilization. After all, what good are ten million pixels of blur? A sharp, 7-megapixel photo is way better than a shaky 10-megapixel image.