When executing the jar file, java has a few extensions that can specify the max and minimum amount of RAM allowed. As mmorton89 has indicated the -Xmx flag is for MAX memory while the Xms is minimum memory . In my experience it's good to keep the minimum around 75% of the max, so if you set the servers max to -Xmx1024m (aka -Xmx1G) then you'd want the minimum around -Xms768m. Helps avoid intermitten lag spikes while the disk is trying to load and unload things to RAM frequently.
CAUTION: Server admins should be aware that they really should never set their FTB Server resources higher than 60% of the machines maximum. So if your server has 10GB's of RAM, 6GB is about where you want it to start and tweak from there. Other services such as the operating system, kernel modules, and in many cases LAMP services all require resources as well, better to under-estimate than over-extend, then crashing the server. This is not a hard rule, just a beginners guideline.
