I've hacked 3 PSX pads so far, the first being a Gamestop pad which is a pelican psx digital pad (no analog sticks) and it only cost 5 bucks! The solder points were nice and big, so they were easy to hack to, and there was also test points which were like big easy, nice, handy cirlces to solder onto. I also hacked 2 MadCatz controllers for my cabinet, and they were pretty easy. All the controls except the up, down, left, right part had test points, and the ones that didn't were REALLY big. the L1,L2, R1, and R2 were also slightly difficult, because I'm not the best at soldering, but it worked great.
Toilet's right, DO NOT use a sony pad, I've opened mine up to see if I want to use it, and they use a way more complicated method of controller input, so it'd be like 10 times harder to solder to, and you'd probably blow out the controller in the process.
I've got a PSX to PC usb convertor, and it works great. The box came with a disc that had the driver on it, actually the driver was for a different brand of controller, a Monster Gamepad, but it worked great. Only cost me 13 bucks plus like 3 shipping. Ebay is the best.
-good luck
-CthulhuLuke
http://www.geocities.com/cthulhuluke -CthulhuLuke's Arcade Parodius