http://bubblemark.com/ has a series of performance tests for RIA applications. Here's the results of my test run tonight on my Dell D820 Centrino Duo 2GHz with integrated graphics.
Silverlight 1.1 = 104 fps (CLR)
Flex (cached) = 64 fps
XBAP = 63 fps
DHTML = 63 fps
Flex = 55 fps