Issue resolved. closing.
Also, you can't downgrade your computer to 16-bit. The last 16bit OS that was released was back in the early 90's
The shear rarity of Software, Driver support, and general acceptance of 64-bit computing for a sometimes unnoticeable performance increase (or sometimes a decrease)? 64-bit computing at this stage in its lifetime causes too many problems then its worth at the moment.
There is no "modes" to run it, You either buy the 32-bit or 64-bit versions. The 64-bit version will be more expensive I bet too.
If you've been holding out on getting a new OS other then XP Hassun, Win7 is the OS you should get, its Heads and Tails better then Vista tried to be and actually does increase performance over Vista (I get at least a 20 frame boost in DirectX10 based games). Micro$haft acually did their homework on this one.
When I switch to Windows 7 I'll go for the 64 bit for sure. Especially because by then the RAM limits for the 32bit operating systems is going to become an issue.