Nah most are compatible with both now-a-days but yeah don't know how long ago exactly but it used to be you had to pick one for whatever system you were using. Other than that they all come with 20pin connector, P4 4pin 2x2 connector, and an Aux connector. I haven't looked at really cheap PSU's but for the most part I see most of them saying P4 and AMD compatible.
yeah, I think they just all come with an extra power cable used for Pentium processors. AMD systems dont use this cable; only the motherboard power cable is needed.
I used a "Pentium 4 Edition" power supply (had this on a red sticker) on an AMD system for a while, before I upgraded to a 400WATT sucker. So it doesn't really matter I think