Im making my first computer and i want to be able to add a second graphics card to boost preformance for feature games and the MOBO i have in mind has a PCI x16 3.0 and a PCi x16 2.0 or will i have to buy a mobo that has 2 PCI x16 slots?
Yes, that won't make any difference. But that board's 2nd PCIe x16 slot is only x4 lanes bandwidth. See: http://us.msi.com/product/mb/H97MG43.html#hero-specific... (Specifications/Detailed)
As a result, adding a 2nd card for SLI will not be practical if you want the best performance from the 2nd card.