Setting -xms and -xmx was a good practice in older versions of Java, when adaptive sizing wasn't working good enough. In Java >= 7 it works really nice, so it's no longer needed. If you need more details you have to read about -XX:+UseAdaptiveSizePolicy
flag or refer HotSpot documentation
Another thing you have to remember about is that you have 8GB RAM on your server doesn't mean that you can use it wholly for Java. You have to left some for system buffers
In Java <= 1.6 setting XMS=XMS and same in case of PermGen is reasonable, but you have to remember about correct tuning of the XMX value, because too much memory will cause cobra effect