550W 80+ I think should be fine to OC your 7870. But at least let us know your PC full spec first, did you OC your CPU?
About Trixx from Sapphire, I believe Vddc should be your GPU Vcore as in the GPU-z.

to OC, just increase your GPU core speed by 10-25MHz step depends on you. Then for each increment, try to run stress test/benchmark like Furmark or Heaven 3.0 extreme couple of times for stability test. If no hang/artifacts/driver reboot etc then you may increase the clock speed again..repeat the step but make sure you monitor your GPU temps and control your fan speed accordingly to balance out the temps/noise level. If you want higher OC, might need to increase the voltage as well..but do this only if you're familiar with OC.

You can also OC your GPU Mem by using above method..do it slowly and steady to find the best stable perf from your GPU. It will take some time for sure..
Seriously I can clearly see the difference in terms of graphics quality between HD78xx/HD79xx compared to GTX670/GTX680..glad that its not just me 
p/s: I'm not AMD fanboy but I'll choose whicever is the best out there. That's why we need to test it out once new gen GC released