I think you've misunderstood what the /3Gb switch is used for. It's completely useless and will achieve nothing if used under a 64-bit environment, the /3Gb switch is to be used in 32-bit OS's to enable certain applications to access over a certain amount of physical memory. The whole point of 64-bit processing is the fact that the addressable memory and the associated registers are capable of seeing/using/addressing larger amounts of RAM.xp 64 or vista 64?
try adding /3gb and (if you have an intel cpu) /pae to your boot.ini
make it so you have two entries, your existing one, and a new one with these switches, that way if it doesnt work, you can still boot