32bit Windows is able to allocate about a gig and a half per program.
For whatever reason, the FTB launcher has capped the amount you can allocate to your modded MineCraft installs at 1gb.
It's always been possible to break the 1gb cap by manually entering in the figures you want as "advanced options", but in my experience, this hasn't really helped all that much with the 1.6.4 packs: I was able to launch the full DW20 pack on a 32bit OS in this manner, but only a couple of times, and the operative word there is "launch"... I was never actually able to enter a world. It wants even more RAM then that, so this sounds like a "too little, too late" thing to me.