The last Amiga show I was at (this must've been about 2 years ago, since there haven't been any in this area for awhile) someone had an a4000 that had a fancy card hooked up to it. Basically, this card was an amd processor with some ram, 2 ps2 ports, usb ports, and onboard gfx and sound. He had the machine running amigaOS and windoze at the same time and could switch between the 2 using hotkeys. From what I understood, this was just basically a small pc motherboard that plugged in as a card so you could share filesystems (?) ect, but you could not use the extra memory or the cpu power on the amiga side.
While this sort of thing looks neat, it's not exactly what I would call an "Amiga upgrade". Hopefully this accelerator is a TRUE upgrade. I want to play some new OS4 games!