I would go with the 64 bit system. It will use all the memory you have installed and is a better system. Check to make sure you CPU supports 64 bit processing. A 32 bit system will only show about 3.5 gb memory and 64 bit will show a lot higher.
32 bit OS's are nearly outdated. You can only make use of about 3.5 gig of memory by using a 32 bit OS. There is absolutely no reason no to go 64 bit. Some older programs have problems running under Vista and Win 7, but honestly, most issues have nothing to do with the OS being 64 bit (32 bit apps will run under a 64 bit OS) the problem is almost always with the new Vista and Win 7 Kernel itself. If you have problems running a program for instance in Win7 64 bit, you are very most likely going to have the exact problem running it under the 32 bit version. The way around this is running the program in XP mode.