The more powerful your rig is, the faster speed you get. Still pcsx2 is a WIP so its speed is not stable. You can get 400fps in menu, 90 in 2D background frame or even 20 in others but if the speed is so high, you can use Limit mode like common emulators. I recommend a C2D/C2Q@3GHz (or higher) and a mid-range vga like 8600GT if you wanna an enjoyable speed and pretty good image quality (1280*720 with texture filtering)