Using keep alive can cause your server to spawn a lot of Apache children which may slow you down more during peak times versus turning it off. If you want to keep keep alive on, I would increase your max spare servers to at least 25 so Apache won't constantly be starting and stopping unused processes.
I would just recommend turning KeepAlive off. That way you save a lot of resources, and there is only a tiny noticable difference in site loading. If you have busy sites, you would probably find a large decrease in cpu usage of apache by turning it off.
For two months I am trying different configurations with various success. It seems that less server load induces more memory usage, so it was a matter of try and fail experience to get the golden mean for my particular hardware. This causes automatic increase of the clients number and memory bottlenecks.
I have firmly established that keeping the MRpChild lower than 2000 creates much strain on the memory, and in fact doubles its use which - in the case of 512 RAM - brings the mashine to a halt and eventually to a series of failures.
When using the standard Apache configuration with 150 clients and unlimited (0) MRpC, then the averahe memory usage is 60-65. But the server load may go skyrocket high (I have witnessed 440 and more) - especially when Google is indexing a combined triple cgi-feed included in mySql dynamic pages. This caused the Apache to fail every 3-4 hrs and restart automagicly.
For the last 24 hours I am trying this optimal configuration for Celeron 2.4/1024 RAM/80GHD: