If only your network port died you can get an PCI or PCI-Express Network card and just plug it in.
What are the other issues you are experiencing? That 7300 isnt overclocked much, you probably have another 500Mhz in there,.
You want to stay under 200 for Motherboard, CPU, and RAM, but have it perform better or on par with a E7300?
I would put your E7300 @ about the same speed as a E7600 in this chart.
From there you can judge what will give you more performance. An Athlon II x4 Would be a decent upgrade, as you are adding two more cores. If you get the right motherboard you can reuse your older DDR2 RAM so you can strech your buget more for a better CPU, Like a Phenom Black Edition