Thank you for your answer.
At the moment I am waiting for more 2 Rpi and I am testing around with a 256 Mb model.
I tested Raspbmc,Openelec and Darkelec as Media Center. I like Darkelec for his fluidity but it is a little bit close as system,Openelec is the same but less fluid. Raspbmc gives more chances to go deeper in system,I changed the skin to a lighter one and it is not bad.
I will do like you said and use the 512 Mb model as all purpose.
On this one I will use a 4 or 8 Gb Sd and a 500Gb Hdd, I have a 32Gb Class 10 45mb/s from Sandisk too which will run the Media Center.
Other suggestions are welcome.
Nr.1 Raspberry Pi Mod.B Rev.2 256Mb Ram (Raspbian)
Raspberry Pi Mod.B Rev.2 512Mb Ram (Maybe i´ll get one)