Check your bios if you have any option to change multipliers (although I doubt there is if its OEM emachines)
Its always better and more stable to overclock in the bios.
As for bottlenecking, do you mean in games?
That athlon x2 wont bottleneck an HD5450, in fact its the other way around.
The HD5450 wasn't designed for gaming, it was made simply for desktop usage (browsing, videos, etc, for motherboards that have really crappy integrated graphics)
Any 2.5 Ghz dualcore...