In order to switch to 64 bit you will have to do a complete reinstallation.  You can't "upgrade" from 32 bit to 64 bit.  The full Retail versions have this option, but I don't know if the Dell supplied will do this.  Most likely you will have to purchase a retail copy of Win 7 to do this.  Dell will only supply what you have, they will sell you a Retail version at the same price as anywhere else, they can't sell you an "OEM" version.   HERE is the Dell Windows 7 64 bit driver downloads. You should be OK to upgrade to 64 bit.   The least expensive way to go is to buy an "OEM" version.  This is the same as the full retail version except, like the Dell OEM versions, it can only be installed on one PC and there is no "free" support from Microsoft for OEM versions.  You still get the same free updates from them, just there is no free support if you were to have a problem with the Windows 7 OS that you couldn't resolve.  The probabilities of needing Microsoft tech support is slim but it has to be noted.    HERE is Win 7 64 bit Home Premium OEM
About a year ago, I bought an Inspiron 1545 with 32-bit Vista Home Premium installed. I took advantage of the $50 offer to upgrade to Windows 7, 32-bit. The box actually included 64-bit Windows 7 as well. I want to make the switch to 64-bit Windows 7. How can I do this? Can I install the copy of 64-bit Windows 7 that I already have, or do I need to have the 64-bit version of Vista to do the upgrade? Can I order a 64-bit copy of Vista from Dell? If so, how much will it cost?  
Switch from 32-bit to 64-bit Windows 7