ATX power supply, What size?

[Piru]

There is no such thing as too many watts though,and basically you can put the biggest rated psu you can afford in it.The A4000 will only take what it needs.

That is a misguided advice. PSU energy efficiency is much worse with small loads. See http://en.wikipedia.org/wiki/Power_supply_unit_(computer)#Energy_efficiency

[Piru]

How many watts Do I need?

300W will be more than enough. It's a common mistake to get a PSU with huge wattage when you don't really need it, so try to avoid that.

[Piru]

well the power supply that I have now is 240 watt...
moreover I made a fast simulation on a internet site that provided some kind of evaluation and it told me that my configuration has an estimated consumption of 435 Watt

That's completely off the mark. Which site was it?

So I think that I'll go for 600 Watt, taking a cheap power supply for now.

Total overkill if you ask me. It'd be much more sensible to get a good quality 300-400W one.

[Piru]

anyway it was a service like this: http://www.thermaltake.outervision.com/index.jsp
remember that I have A LOT of 3,5" devices!

Not really. I ran 6 3.5" HDDs, ZIP-100 drive and a CD-RW with a 300W PSU.