As long as there are Dell drivers for 64 bit (minimum Vista 64 bit if there are no Windows 7 64 bit drivers) go for the 64 bit.  Most new systems (from all PC vendors) are now being sold with 64 bit OS, so that has become the "standard".  64 bit will load a little faster and with some programs run them a little faster.  If you have 4GB of RAM then it will be fully addrssed and used by 64 bit OS (32 bit cannot fully address 4GB).  
I am currently running Vista 32Bit but my system is 64bit compatible. Has anyone made the jump to 64bit after running 32? I know I can not upgrade as I will need to do a fresh install and thats ok as I need a cleaning anyways.  Just wondering what I would gain.   
Switch from Vista 32 to Win 7 64?