As far as I'm aware, no program will use your 18 Gb, it's not just overkill, it's plain silly...
This rig here's doing fine on 32bit with 4.5Gb RAM. Just need to get a new GFX card and it'll be a pretty good PC, seeing as it's handled everything I've thrown at it so far...
As far as I can see though, RAM has little bearing on performance as much as your processor or GFX card when it comes to games...
If you have such an amped up PC, why not move onto a 64bit Development engine like CryEngine, Source or UDK. You'll have wasted all that money for next to no benefit otherwise because the limits are in FPSC, so it'll perform just as well for you as before anyway.
Your signature has been erased by a mod please reduce it to 600 x 120.