My current pc has:

cpu: amd 955 be
ram: 16 gb ddr3 1600mhz
gfx: sapphire 6570

Back in cata, I was able to run 10 cata clients alright with all clients on low, main 30fps rest 15fps, but occasionally the main would freeze for a second, I know my cpu would be very close to maxing out. But I tried multiboxing again recently and mop seems to use a lot more resources, 10 is close to impossible with my low end computer. What would you recommend I upgrade first (or everything, for that matter)?

My graphics card definitely needs to be replaced.

Thanks.