Means you are likely running 64bit hardware with a 32bit OS. If you want to run Windows 7 64bit, then you need to have 2GB or more of RAM and make sure you have all the 64bit drivers and software for your board. There is nothing wrong with running a 32bit OS on 64bit hardware. The 64bit instructions are just to allow for more RAM and let the programs be able to take advantage of using more RAM than the 2GB program limit. The only reason for you to switch to a 64bit OS is if you really need more than 3GB of RAM. If you install 4GB or more RAM on a 32bit OS, you will only see 3.3 or 3.5GB of RAM available to you. The 32bit instructions just can't address more than 4GB of memory. Also, the only reason 64bit machines feel faster is because you are opening up the physical RAM limitations and the operating systems 2GB application limitation. 64bit instructions can feel a little snappier because Windows Vista 64 and Windows 7 64 are optimized to take advantage of this new bandwidth.
If your system is working, and you have no specific need for a 64bit OS, then don't worry about it. It will not make anything any different for you. If you install a 64bit OS, likely most of the software you are running is still 32bit. Just the OS is capable of managing more 32bit applications because it doesn't have the physical RAM limitations. Take a look at some sample applications like Firefox, Skype, MSN Live Messenger, Yahoo... etc... they are all still 32bit. So even if you are running 64bit, there is no benefit to these programs. If you want to run Photoshop CS4 or 5, yeah... it has a 64 bit version and works faster, but most of the 32 bit filters I want to install will not work unless I use the 32bit versions.
So the simple explanation is if you don't need more than 3GB of RAM or have less than 2GB now... then you have no reason to install a 64bit OS.
Do you require use of a 64bit program that you cannot find a 32bit version of? 90% of all the programs out there are still purely 32bit!