My personal opinion, and the strategy that I implement, is full fault tolerance and redundancy. Any combination of the following can happen at any moment while you're in the middle of a trade:
* You could lose your internet connection
* Your computer hard drive or power supply could crash
* You could lose power
* When you need it the most, your UPS could fail to work because you waited to long to replace the battery when it started going bad (or it could just be faulty)
* Your monitor(s) could die
* Your broker's servers could go down making it impossible for you to submit and/or cancel orders
So I always make sure I have two of everything:
* Two computers with different monitors attached
* Two UPS units
* Two different internet connections using two completely different technologies (cable & dsl);
* Two routers
* Two trading accounts with two different clearing firms (both connected and ready to trade).
That way even if one entire setup spontaneously combusts, all I have to do is turn my head and I am ready to go. Also, if my clearing firms servers go down, I can immediately hedge off my position in a completely separate account with a completely different firm that has a completely separate server farm.
The only single point of failure that exists is if trading is halted because of an exchange outage. There isn't much you can do about that though other than maintain a backup account at a firm where you can trade an appropriate vehicle to hedge with that is traded at a separate exchange (like hedging off your ES position with SPY if the CME goes down).
And both computers don't necessarily have to be dedicated to trading. I dedicate one computer exclusively to trading, and the other computer is a recreational computer that I use for everything else. However, on the recreational computer, I jut run a virtual machine that has my trading applications in it, that way its completely isolated and unaffected from any problems that may exist on the host machine (like a virus or a worm). I just start the virtual machine when I start trading, and suspend it when I'm done. But this way while I'm trading, I can always get to the markets even if one machine has a major meltdown.