Total number of concurrent connections. I suggest using this formula to determine the right value for your server.
MaxClients = 150 x RAM (GB)
This value should be same as MaxClients for good performance.
MinSpareServers and MaxSpareServers
Leave those values to:
Controls the number of request the a child serves before the child is killed. This should not be set too low as it will put an unnecessary load on the apache server to recreate the child. I suggest setting it to 0 for more than 2 GB RAM
KeepAlive and MaxKeepAliveRequests
KeepAlive provides long-lived HTTP sessions which allow multiple requests to be sent over the same TCP connection. In some cases this has been shown to result in an almost 50% speedup in latency times for HTML documents with many images, but having keepalive on is also a resource intensive setting.
Sets the number of child server processes created on startup. If you run low traffic websites on that server set it low to something like 5. If you have resource intensive websites on that server you should set it close to MaxClients.
The default value is 300. You should set time to something a bit lower. A setting of 150 is probably ok.