Ok. First you need to back up all of your important data to removable media, like an external hdd, flash drives, DVDs, ect.
You will lose all of the programs you downloaded or installed from a disc. If you still have the installation discs for the programs you can reinstall these after installing the new 64-bit operating system. If you paid for the programs you downloaded and haven't changed you email address you should be able to download the programs and activate them. You may need to contact the vendor in order to do this.
You may find that some programs were intended to be used with the original 32-bit operating system, there isn't a whole lot that you can do about this. You may be able to run these programs in compatibility mode successfully.
Hope this answers your questions.