1) AM3 is really outdated, so unless you can buy on EBay forget it.
2) Bottleneck will vary by the game, but it will be HUGE for many games.
How huge?
An FX-4300 for CPU intensive games with an R9-290 (say Skyrim) is as low as 65% of the performance of a good i5/i7 Intel CPU like the i5-4690K.
Not sure what CPU you have but here's some per-core comparisons:
i5-4690K: 2234
FX-4300: 1411
Athlon II X3 450: 1062
So, it's conceivable that for some games you'd get about HALF the frame rate to what others can get if your CPU is similar.
What you SHOULD do depends on what your budget is, games you play, and what parts you currently own. Even with a severe bottleneck you may have enjoyable game play.
The system however is a dead end so I...