Your CPU and motherboard are pretty old, but they were pretty good when they came out. So we should not worry about replacing them just yet.
For a new GPU I would either go with the
MSI R9 280 for $184.99.
The
XFX R9 285 for $199.99 after a $30 rebate, if you wanted a little more power.
If you want to go Nvidia, then the
EVGA GTX 760 for $179.99 after a $20 rebate.
The 760 does not have as good as price-to-performance ratio as the AMD cards.