It does not matter : I will keep this computer running for years. If I must buy a new case, it's not a problem. But it must offer at least 4 HDD slots, 6 if possible.
This is very important imo, for example you might be happy with your current case performance/noise
Noise was a problem when I bought my "barebone server" (the P4 I am still using). I was living in a small house and was sleeping very close to it. Today I live in a quite big house, and this server is located in the garage, with the cars
Really : performance per watt is the only thing important for me. And since often those cases are silent...
The problem with the upgrading solution is that my barebone is using old (P4) power supply format, and I cannot replace easily the power block. It will be easier to buy a new one.
I'll have a look at all your suggestions, thanks !
About Atom : I don't know where to buy one, and don't know how efficient they are.
The actual CPUs are very efficient but it doesn't matter much unless you aren't planning to use hard drives or other power-hungry peripherals.
Atoms are usually a cost-saving choice.
In fact the CPU power is not really important. My Pentium 4 stills gives me satisfaction. The only thing I'd like him to do that he cannot is transcoding. But I guess any modern CPU with multimedia instruction set will be able to do that ?
And still this box is going to stay in idle state 98% of its lifetime : I am looking for the less hungry cpu in idle-mode. Power consumtion when loaded is fas less important.
Btw : thanks for info about ARM and I/O : I give up on this idea