How can two servers balance load by network?

We had an overload problem on a server and decided to use another old computer as a back up, they are both working on different ips,now I was thinking of using nat to balance traffic between the two but it doesn't seem like the best way to do it.

What is the standard option to balance load between these two ips? or what do you use? I tried using cisco SLB but my 6500 won't accept it.