It seems you're misinformed about 32- and 64-bit systems...
I'm going to assume your OS is 64-bit to start with, since you said you run COD 4 in 64-bit.
Whether or not a game fully supports 64-bit makes little to no difference as Windows only allows applications to use 2GB of RAM (I might be wrong on this point, but I'm pretty sure I'm not). Also, 64-bit isn't any faster than 32-bit. If you're on a 64-bit OS it's marginally slower running 32-bit applications than a 32-bit OS would be, but the difference is negligible.