Eventually I decided to buy a 30Gb, which is still more convenient in terms of cost/mb. I'll eventually replace my 12Gb with a 60Gb later on next year. So far 42 is enough. So far... Wow, you really never have enough disk space. I always believed so and I always get a confirmation of that...
Ok, soon I'll upgrade my empeg and I'll keep you posted :)
Thank you all for your help, as usual!