Hmmmm... to take a guess (being safe that is), typically you would have 2 or 3 4pin molex on one rail, which would can hook up 2 HDs and 1 DVD/CDROM. Basically that would be around 20-30W per HD or DVDROM.
Soooo... 60-90W roughly on a single rail. If you have fans that are rated 3W a fan, to equal 60W that would be 20 3W fans. . o O (just the thought of running 20 fans)
As I stated in my first post, for fans only use 3 together on one connector, and only 4-5 on 1 cable. Also that is only if there are no major components on the that cable, CD/DVD drives, HDDs, floppy, extra cards, motherboard, anything that is not a fan or cathode really. I use a PC P&C 750w, so Im using the best for that wattage and one of the absolute most reliable power supplies money can buy, and I am still careful about this. If your system is using 10 or more fans, then a lot of people use 5.25" power supplies and hook up 3-5 fans to them so you are not hurting the rest of your system.