Which motherboard do you have? I ask because if you already have the GTX 950, there's little point in getting an A10 APU if the motherboard can support something like an x4 860k (which should also be cheaper).
As a pure CPU then the x4 880k would be 'best' as it has the highest stock settings. Should be cheaper than the APU equivalent. I would suggest checking prices where you are. There is the possibility you will need to update the BIOS to make sure the new CPU (or even APU) will work with the motherboard.
Alternatives would be the newer architecture of the x4 845 (which doesn't overclock) or even the x4 860k.
EDIT: forgot to ask. What processor do you have now? Depending what it is it may not be a worthwhile upgrade.
No, you don't need a new motherboard (and if you did you may as well look at saving more for a more substantial upgrade). For confirmation I linked ASUS's own CPU compatibility page in my last post and the processors I've mentioned are all on there.
What I don't know is the BIOS version of your motherboard. The CPU compatibility page lists the BIOS version required for a particular processor to work on the motherboard. There is the possibility you'd need to update your BIOS with your current processor before installing the new one.
EDIT: the x4 860k should be better at multi-threaded tasks. How much difference it will make will depend on the software you're running. If current performance is unbearable then by all means take the plunge and upgrade. But financially I would be tempted to continue saving up for a more substantial upgrade.