what is the best CPU to run in a MSI 790FX-GD70 motherboard. I currently have a AMD Phenom II x4 955

Solution
The AM3 socket was not current for long.

The best chip made for that socket was the Phenom II x6 1100T. New, you could find them for $200; but they're only available on E-bay now... for $170-$200 used. The 1100T could be overclocked quite well, so with a good CPU cooler, and a solid motherboard you could get pretty good performance out of it.

However, for the price of that chip, $200 or so, you could get a FX6300 AND a new good motherboard to run it on, or spend a bit more and get a Haswell i5 and motherboard,

What you have is a decent chip. There is no 'super' upgrade available.
The AM3 socket was not current for long.

The best chip made for that socket was the Phenom II x6 1100T. New, you could find them for $200; but they're only available on E-bay now... for $170-$200 used. The 1100T could be overclocked quite well, so with a good CPU cooler, and a solid motherboard you could get pretty good performance out of it.

However, for the price of that chip, $200 or so, you could get a FX6300 AND a new good motherboard to run it on, or spend a bit more and get a Haswell i5 and motherboard,

What you have is a decent chip. There is no 'super' upgrade available.
 
Solution