before doing this i could only use 700mb but with Jrockit MC i can use 1800mb! (and i was using vista with 3gb of memory)
first install Jrockit mission control : http://www.oracle.com/technology/software/products/jrockit/index.html
DONT FORGET to download the version that is right for your java version!
then edit knime.ini and change
-Xmx1024m or any other working number, every time it loads quit knime and push the number higher.