As DLow said it doesn't really matter. 32bit should be fine generally on linux. If you have Windows and 4GB of ram and you install CentOS 32bit then you will lose 0.8GB of ram. Also, rarring/unrarring of files bigger then 3,2 gGB of ram requires 64bit system. I also thought about uploading/downloading of files bigger then 3.2GB but I don't know if it's true.