The performance of 970 and 990 is identical
The only difference is the option to crossfire with a 990FX at 16x/ 16x or crossfire at 8x/8x with 990X
But the MSI G46 motherboard is suspect . It replaced the G45 that caught fire and burned a few times .........
IMO buy an ASUS M5A97 which is the best of the 970 boards http://www.newegg.com/Product/Product.aspx?Item=N82E16813131873
or buy an Asrock 970 extreme4 which actually has a 990X chipset in it . Its not as good quality as the ASUS but its better than the MSI http://www.newegg.com/Product/Product.aspx?Item=N82E16813157262