What's the point of running SL in 64 bit if you don't have more than 32Gb RAM? It's not going to run any faster since all the apps run in 64 bit mode even if you startup in 32 bit...
On point of correction here. Im running a hackintosh in 64bit mode with all kexts loaded as 64bit. You dont need more than 32GB Ram for 64bit to have and improvement of speed over 32bit. There has been an improvement in speed and a noticeable one too - programs start quicker, are much more snappier and it literally takes less than 3 seconds to shutdown.
Im running a 2.8Ghz core 2 duo with a 9800GT 1GB VGA and it beats my old Core 2 quad 2.4 hands down (did a bench mark test using Geekbench) - Snow Leopard 64bit VS Leopard
So, with only 4GB of Ram, just by changing over to 64bit DOES improve the speed drastically. If you are running a Mac you wont see a difference. Remember running a hackintosh also disables the "handshake" between the OS and hardware.