In my case it started to work only with
java -Xmx1G -Xms1G -jar Tekkit.jar nogui
All other options (3G,2G ; 2G,1G ; 1450MB,1G) gave me error messages.
After starting, it displays a lot of messages and stoped at "Galacticraft remote version found: 0.1.39"
Then I had to close the server, restart it and now it works.
OS=Win7 x64, 12GB RAM, Java 32 bit version 7, update 25 (build 1.7.0_25-b17)
Yes, I'm running a 64bit OS and 32bit Java. Checked at Oracle and that's recommended in most cases. Is that a problem for the Tekkit server?
Hope this helps.