There aren't many "tweaks" you can do to those boards. The chip is soldered in, and I don't think they overclock well. Like Will pointed out, some videos go down to 3fps in complex scenes. The only way around that is to reencode all the video into something easier to decode, and that's rarely worth the time/effort. Besides, that's way too much ram... go with 512 and spend the other cash on a faster CPU.
As for physical size... yeah, our rig is about a square foot in size. It's running in the box the MB came in.