I'd choose 64 bit operating system with 6GB (or more) RAM.
The basic criteria on deciding weather to go for 32 bit or 64 bit is the power of your computer. If you have good processor and good memory, there is no reason to install 32 bit OS.
But if you have a PC with lesser processor (core 2duo or less), 32 bit is the way to go.
Overall, 64 bit is much faster than 32 bit in good CPU computers.