This idea gets brought up on a regular basis.
Personally, I think it would be easier to create an FPGA based board that would plug into an X86 PC and emulate an advanced Amiga.
I'd be amazed if an FPGA that has to communicate through a "slow" PCI-e bus could run faster than pure software.
On a low end PC you'd still have 3 to 7 multi-GHz x86 cores plus the GPU to act like a custom chipset.
