Another "dreamer" thread thats going nowhere then eh?
AGA struggles with 640x256 @ 256 colours (8bit colour resolution), "normal" HD is 1377 x 768 (there are slight variations too) in 32 bit colour, "full" HD is 1920 x 1080, the AGA chipset would need to be totally replaced, what you would end up with is only an Amiga in name. Others have already tried this - see Amiga One from Eyetech.
The problem with moving the Amiga platform forward without a company with any money behind it and sound management, is that we are only left with "dreamers" and not anyone who has the money or forsight to make a logical sustainable strategy for this once great platform. Its such a shame

we have individuals like Jens and Piru et al who are so knowledgable and talented, and a lot of good intentioned people like the original poster but we get nowhere in this world with only those things :/
Until Amiga rises from the ashes, buy a Sam440

Ive read reviews of it from people outside the community who say its great with OS4 (just a bit expensive) but it has the hardware to get you much closer to an Amiga with HD today
