Ajamees

Hey. Is there any way to allocate ram like 256MB, because my computer is old and tekkit cracher often. With palying original minecraft i have allocated ram 256MB and it wont crash at all, but when i but there 512MB ram it crasher sometimes. My computer has 756 MB of RAM. I hope there is any way to allocatwe only 256MB of ram. (Sorry for my bad English)