Any intel combo would probably give a slight performance boost over the phenom 955BE. Under 300$ you're most likely looking at a non overclocking build, if you care for that.
If you're okay with a locked cpu, one of the most cost efficient would be:
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($184.98 @ OutletPC)
Motherboard: Gigabyte GA-H97M-D3H Micro ATX LGA1150 Motherboard ($79.99 @ Newegg)
Total: $264.97
The motherboard doesn't really matter, because it is a locked CPU, you could go with a H97 or z97 depending on your needs.
If its no huge rush, and you feel like you could ride out your cpu, get an aftermarket cooler and overclock it if you haven't already.