Well it's never bad to keep some distance to a PSU's limit but your recommendation of 500-750 watts (!) for ventilator's system is totally off the scale. You'd have to recommend 800+ watts for someone with a really decent graphics card and SLI wouldn't start below 1000 watts when you choose such a high range to recommend...

All you need to make sure is to buy a quality PSU though: You of course will have experiences of "not enough" watts when buying like a 500 watts PSU from LCPower which will die when delivering something around 350 watts (Planet3DNow "loves" reviewing them ^^). If you do so you can even power a High-End Single-GPU gamestation with something around 400 watts. I myself e.g. run a Phenom 9850 with a HD4870 and the usual stuff with a 420 watts PSU and that only because the 380 watts version was out of stock when I bought it...

The point about hard-drive power consumption has some truth to it but you have to keep things in perspecitve - HDDs still drain little in comparison to a GPU. I'd use 10 watts as a guide for standard HDD power consumption. If you use some high-speed server HDDs then this might go up to like 15 watts each. So yeah if you have like 6 HDDs in your PC that's definitely a fact you should consider...

Enjoy your meal
Toast