More RAM for modded minecraft server?

[Xenphir]

Well, me and my friend started to play together on my modded hamachi server. However, the lag is intense due to it only using around 512MB. I've looked up ways to add RAM and one of them was to make a .bat file with some java code inside it to launch it with more ram. The issue is, this only seems to work for vanilla and I have no idea how to do so with minecraft forge universal 1.6.4. Any help would be greatly appreciated!