Your power supply has more than enough juice, both in watts and amps on the +12v rails.
To answer your questionss
1) if your PSU can handle it (which yours technically can) it's a good idea. If you had to upgrade your PSU in order to run SLI then it's usually more beneficial to sell your single card and buy a more powerful single card. The cost of a PSU + single card is usually not worth the investment.
2) probably, it is an entry level PSU and not really made with high end overclocked gaming systems in mind. But technically speaking, it has the power.