What are you going to mine? Mining BTC/LTC (using GPU/CPU) is not very profitable now.
Anyway:
- how do you link two graphics cards to mine ?
[...]
-is there a trick to link them to work together ? or just have to switch them in PCI-e slots ?
SLI/CrossFire, no tricks, just read the manual.
- wich one to link on the monitor ?
Doesn't matter AFAIK
- 2 x 7870 gfx cards enough to start mining as a noobie to understand how it works ?
Which coin?
- what s the dos command to launch Cgminer ? never used dos before
It's not DOS command, it's Windows command prompt command. Anyway, I can't tell you as it depends on which pool are you going to use. (usually, a pool's page provides you with the command)
- is there a list of coins mined with GPU ? (and a list of coins with CPU ?)
https://en.bitcoin.it/wiki/List_of_alternative_cryptocurrencies