Values for memory are most stable when they equal 2 to the power of n. Currently, the slider for allocated memory increases in stages of 48MB (which is 2^5 + 2^4).
With the current slider, the only exact GB numbers that can be reached are 3GB (3072MB), 6GB (6144MB) 9GB (9216MB) and 12GB (12288MB).
Either increasing the slider to 64MB per stage, or decreasing to 32MB per stage will allow users to select by the gigabyte. Users may also appreciate the slider showing both MB and GB values.
The JVM arguments will also accept whole GB values to allocate RAM à la -Xmx1G as opposed to -Xmx1024M