CPU is the most important here, then graphics card. Dolphin will only use two cores, in theory a fast and/or overclocked Core 2 Duo would be sufficient for running the decently compatible games with a playable speed. Core 2 Quad or i7 won't hurt though. Graphics-wise, you could get away with a decent PS3.0 card to cover all bases. Feature set is a bit more important than raw speed for emulation, although at higher resolutions, a bit of extra GPU power does help quite a bit. RAM won't make a lot of difference really unless you have a really small amount or something. If you have 2GB+ DDR2/DDR3, then you're good to go.