Hello,
Please don't even think of an FX, you have already a better CPU (an FX6xxx struggles most of the time even opposed to an i3).
As Karsten75 said, maybe you choice of the motherboard holds the system back (an SATA3 SSD on a SATA2 port cannot be considered a bottleneck, unless you have an old SSD and its capacity is almost full).
What kind of problems do you have with your system anyway? Please list the specs (mobo and PSU especially).