I have a 2.8GHz AMD dual-core, 4GB of ram, a GTX 285, three hdds and two optical drives in my pc. How big of a PSU do I need?
The same Tagan 480W PSU I've been using for ~ 5 years.
Just because your 500W psu didn't have enough power connectors doesn't mean 500W isn't enough. Just make sure you have one capable of sustaining its max output, instead of only being able to sustain half of what is on the box (and being able to do 1ms spikes of the power output they advertise). The 80% and 85% efficiency certifications are a great way to confirm this, as well as getting you an efficient PSU.
I can see PSU requirements being much higher for multiple graphics chips (the top end ones use 200W when going flat out), but do you really need multiple (for example) GTX 285s? I run all my games at max res, with HDR, AA, AF, and all the game's own options on max, and I can still put vsync on afterwards because the framerate's so high. And that's on one GTX 285.