Zombie processes don't increase the load on your server; in fact, they do very little at all (nothing, in fact) which is how they get their name. A zombie process is simply a process that finished, but who is waiting to pass on their exit value to the parent process that created it. If the parent process never collects the exit value, then the children stay in the zombie state; harmlessly, except for holding their process ID.
Your load problems will be caused by something else. Top is not an accurate way to measure process resource consumption; it's only useful in cases where a single process is using all or nearly all of the shared resources, because an accurate view of resource usage cannot be gained within a second. The load average is more likely to be correct, as this is measured over a period of time.
Look at the output of 'ps -aux', and scan down the 'Time' column. Ignore system and kernel processes, focusing rather on the applications you run on the server (web servers, mail servers, etc.). It may be the case that none of these show abnormal levels of activity, but rather that the load is generated by continually-spawning PHP or CGI processes, for example, from client activity. The rate of requests generated in your logs may help to indicate the cause of this.
I guess when I was seeing a lot of zombie processes when the load was going high the opposite was happening, because the load was so high the processes were not exiting quickly hence the high number or 'zombie' processes.
ps -xla << will tell you which processes are Zombied so you can kill them.
There is no doubt that more RAM will bring down the Server load. Although Zombies do not attribute to the Server load, they are indicative that something is not right -- and the higher the number of them the more something is not right.
To get a better handle on the situation you must become one with your Server. ohhmmmm ... ohhmmmm
It's late, what can I say.
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