You could easily have gotten an answer using Google. Google is excellent. You should use it.

http://windows.microsoft.com/en-US/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions#

You will have to do a complete new install.  You can't upgrade from 32 bit to 64 bit. 

Before you do, backup all your user data to some external media such as a USB flash drive, as you will want to reformat the hard drive and do the install and when you do that, all will be lost. 

Along with the Windows 7 64 bit DVD, do you have the Dell drivers and utilities disc?  If not, you will probably need some of the Dell drivers as Windows 7 will install some device drivers but they will not be full function.  e.g. the sound driver needs the Dell driver for proper operation.   The 64 bit Dell drivers are HERE

Basically on an install the correct and required install sequence is (1) Install Windows (2) Install Dell Laptop System Software (3) Install motherboard chipset drivers (4) Install Device Drivers including video, Dell sound, Ethernet, touch pad, etc.   As I noted Win 7 will install a lot of drivers but on devices that do not work or do not work correctly you will need to install the Dell driver.

After you get the OS installed and all the basic items installed, you can then start to reinstall your applications, user data, etc.   If you don't have an antivirus program, the free and highly rated Microsoft Security Essentials will work well.  Click Me