[jitter] [OT] video cards in current imacs

I am in the market for a new machine.  I do intensive audio/video/3D work, and have no experience with the new intel macs.  I have been delving into shaders more and more, though many things do not run so well on my stock video card.  I have been following the threads on all the different formats, etc., and am mostly wondering about the performance of the ATI Radeon X1600 graphics that is in the 20" imac.  Does it support all the new stuff?  Also, what is the benefit of 256 megs of VRAM over 128?  I assume that means twice the memory for texture storage, but does it result in any speed improvements?

And on another note.  Is there anyway to upgrade the videocards?  I would love it if I could get an imac to drive two EXTERNAL displays.  Any other pointers about these machines would be appreciated before I drop $1.5K.

Thanks,
Christopher Overstreet