I used to emulate an Amiga 600 on a 1.25GHz Athlon XP and it was fine. Recently I used a 1.6GHz Atom netbook to play Zerberk on a plane at 36,000 feet.
Multiple cores won't help Amiga emulation as WinUAE doesn't take advantage of extra cores. WinUAE's developer said it would be very difficult to take advantage of the extra core and still keep timing accurate.
Any modern graphics card should have no trouble handling WinUAE, especially if you're just running old games and software. I don't think WinUAE relies too heavily on the graphics card.