The first question is how many accounts do you want to box at once?

The second question is what is your goal end game? PVP PVE or both?

For 5 accounts you can easily do them on one machine, with the right spec's it's even pretty damn smooth for most cases. I would say that I have polished all the glitches out of my software system but i'm still considering hardware when it comes down to actually PVPing in arena's. I'll know for sure once my shammies hit 70 and I get a chance to actually do some pvp with them. The limited PVP I've done is a few BG's and some world PVP (PVEserver) so it's far too early for me to be the judge.

I don't however know of any top ranked PVP boxers using a software solution... I could be wrong though.