a system thats equivilant to the console games that your playing offcorse.
I say a sufficient Core 2 Duo, Core 2 quad, Intel I5 /i 7, Phenom 2 X4/3/2 , Anthlon X2/3/4 with 3gb of ram + Nvidia 7900GT +, ATI Radeon X1900XT +
Emulation requires hardware a lot better than the console. I can emulate an NES system on a NDS but I doubt I could emulate it on a gameboy. To emulate the NDS properly for some games, like pokemon, which are more demanding, you need more than an athlon 4400+, which is multitudes better than the ARM processor already. You would need a decent modern gaming rig in order to emulate even the ps2.