if you run it with a batch you can do this for the server:
"c:\program files\java\jre7\bin\java" -Xmx1G -Xms1G -jar Tekkit.jar nogui
and this for the client:
"c:\program files\java\jre6\bin\java" -jar Minecraft.exe
so you use different paths for jre6 and jre7. have fun!