new graphics card

Solution
Yes. You will be cpu bottlenecked and the psu with that pc will not be enough to run the 280.

If you have not already bought the pc, don't buy it. If you have bought, upgrading is going to be expensive. IMO you will need new m/b, cpu, psu as a minimum.
Yes. You will be cpu bottlenecked and the psu with that pc will not be enough to run the 280.

If you have not already bought the pc, don't buy it. If you have bought, upgrading is going to be expensive. IMO you will need new m/b, cpu, psu as a minimum.
 
Solution
The cpu is not the primary bottleneck - it's the psu. With your stock psu, an R7250 or GTX 750 would be about the best you can do.

If you're willing to upgrade the psu to a solid 650-750w unit, then the 280 will work with minimal issues on the cpu.