Er? Current PCs can far, far exceed PS2 games' performance. The Xperia Play exceeds it's performance. The PS2 was 300Mhz with 32MB RAM, granted these can't be directly compared but my 1Ghz 576MB RAM HTC Desire is much faster.
If you're thinking of PS3 games then PCs are tested a lot more but still exceed it. Poorly ported games can suffer performance problems but they are trying to do more than just run a direct copy. For example, I'd run them at 1900x1200 whereas the PS3 runs at 1280x720 or less (even if it claims to be 1080p, it's usually 720p at most).
I think there were emulators, but Sony lawyered them away so their PlayStation certified thing wouldn't have competition. I'm not sure how/where you get the actual games anyway.