Thank you for your quick replies! I think I will give it a try. The $0.28/kWh are just estimated, as I honestly don't know what I (will pay). I should probably ask for a bill from my landlord (what a strange word). 
Will those two 5970's have to be from the same vendor (for cross-fire)? It seems hard to find two cheap, used units from the same vendor.
One general question, if you don't mind: I also read in some other threads about using USB flashdrives for those rigs. Are they used instead of a hdd? Or are they used to store the wallet? If used for both, isn't it very risky to use them, as they have limited reading/writing capabilities (hence data loss after using it for a while).
I think FPGAs are not what I search, the GPUs may still be sold to other miners/gamers or used for rendering. And imho the costs for getting a decent Mhash/s rate on FPGAs seems pretty unrealistic.
Another question, just happened: I received a payment to my local wallet for mining, but in my Bitcoin wallet the address disappeared. I think it's not supposed to disappear, is it? I should be able to receive unlimited transactions on one address. What happened? 
nvm, I got it all wrong.
Will those two 5970's have to be from the same vendor (for cross-fire)? It seems hard to find two cheap, used units from the same vendor.
One general question, if you don't mind: I also read in some other threads about using USB flashdrives for those rigs. Are they used instead of a hdd? Or are they used to store the wallet? If used for both, isn't it very risky to use them, as they have limited reading/writing capabilities (hence data loss after using it for a while).
I think FPGAs are not what I search, the GPUs may still be sold to other miners/gamers or used for rendering. And imho the costs for getting a decent Mhash/s rate on FPGAs seems pretty unrealistic.