2 GHs of good matched RAM, newer 256MB graphics card, and a CPU comprable to a minimun of a P4 2.8 or above should be good. But a 3.0 or higher, or a Dual core would be better. Plus keeping Windows XP as your OS helps too.
I'm running a P4 3.2E, 2 GH of matched DDR RAM, a XFX 6800XTreme 256MB DDR3 256 bit card and frame rates seemed fine with SH4 when I had it installed.