I have a useless Nvidia, until now, unconfigurable to mine (7300)
So, if I mine just with CPU, can I get more coins mining others coins like NMC,LQC,LTC,SC, or whatever, and trading after for BTCs?
This is largely what the sCrypt() based forks are intended for - both utilizing un-tapped CPU power on dedicated rigs or allowing casual miners to use their CPUs. At present, the difficulty on the Bitcoin network is so high that it's not even a question of profitability but capability - even with pooled mining, CPU mining is so slow that you may not actually submit even a single share for each round. That said, let's do the math. I'll use Tenebrix (TBX) as an example since I have numbers handy for it.
Keep in mind that the numbers presented are based on difficulties and exchange rates right now and as such will change. I will provide sources for all values and calculations so that you can re-calculate at any time.
A decent dual-core CPU will net you about 3 kH/s on Tenebrix (0.003 MH/s) and our friend the Bitcoin Mining Calculator says that at the current difficulty of 0.288 you should mine 10.48 TBX per day. BTC-E has the current TBX/BTC exchange rate at 0.000195, so that's worth 0.0020436 BTC per day. The same math could be done for FairBrix, LTC or any other sCrypt() derived forks.
Now for the BTC side of the fence: At the current difficulty of 1379647.43921 (auto-populated in the calculator, but available from the same blockexplorer clone I linked earlier) the same CPU will run about 3 MH/s on the Bitcoin network worth approximately 0.00219 BTC, roughly the same result.
So there's your (hopefully future-proof) answer: let the math guide you. As of this exact moment, mining TBX about the same efficiency though you'll probably see smoother returns with an sCrypt() fork (i.e. more shares per hour)