This is what is use on my server
java -server -Xincgc -Xmx6G -Xms3G -Xmn2G -XX:NewRatio=2 -XX:CMSFullGCsBeforeCompaction=1 -XX:SoftRefLRUPolicyMSPerMB=2048 -XX:+CMSParallelRemarkEnabled -XX:+UseParNewGC -XX:+DisableExplicitGC -Xnoclassgc -XX:+UseFastAccessorMethods -XX:CMSInitiatingOccupancyFraction=90 -XX:+UseConcMarkSweepGC -XX:UseSSE=3 -XX:+UseCMSCompactAtFullCollection -XXarallelGCThreads=2 -XX:+AggressiveOpts -XX:MaxPermSize=512M -XX:ReservedCodeCacheSize=1G -cp ftbserver.jar -jar ftbserver.jar
java -server -Xincgc -Xmx6G -Xms3G -Xmn2G -XX:NewRatio=2 -XX:CMSFullGCsBeforeCompaction=1 -XX:SoftRefLRUPolicyMSPerMB=2048 -XX:+CMSParallelRemarkEnabled -XX:+UseParNewGC -XX:+DisableExplicitGC -Xnoclassgc -XX:+UseFastAccessorMethods -XX:CMSInitiatingOccupancyFraction=90 -XX:+UseConcMarkSweepGC -XX:UseSSE=3 -XX:+UseCMSCompactAtFullCollection -XXarallelGCThreads=2 -XX:+AggressiveOpts -XX:MaxPermSize=512M -XX:ReservedCodeCacheSize=1G -cp ftbserver.jar -jar ftbserver.jar