Anyone with the dual-core E-350 should benefit from the new Flash Player 11.2 Beta, which introduces support for multi-threaded video decoding. This will not improve video decoding performance on the E-250 as it is a single-core chip. Anyone who has been following the articles on Phoronix will probably know about this already, but I thought I'd mention it just in case. Flash Player 11.2 Beta can be downloaded from Adobe Labs at http://labs.adobe.com/downloads/flashplayer11-2.html
My feeling is that flash performs scaling on the CPU rather than the GPU, on both fglrx and radeon. This positively kills performance, even though the CPU itself can decode 720p just fine (as evidenced by VLC and Totem). Threaded decoding will also help a lot, but not if the scaling is left on the CPU.