32-bit exists right now because its been around a while. 64-bit has been around for a little while, but not long enough that the common PC user's 32-bit computer warrants an upgrade. The difference between the two is like looking at a freeway/highway. Each freeway lane represents a channel or path. Having 32-bit is like having a 32 lane highway (this is up from 16-bit & 8-bit in the past). So 64-bit develops and we have double the amount of lanes, which gives the computer even more room to pass data quickly.
The major setback with 32-bit that I see is the limit of RAM you can use on your PC (less than 4GB), whereas 64-bit has the ability to handle much more... which makes multiboxing much easier on the computer itself. I just passively keep in touch with new hardware, but wouldn't be surprised within 4 years that 128-bit PCs are common (pending any physical limitations). FYI, when 32-bit was fresh and new, no one dreamed of hitting the < 4GB RAM cap for low cost. I remember how excited I was just to break 1GB. Now my 6 month old PC is sporting 8GB & plenty of peeps are using even more than that.

As far as upgrading from 32-bit to 64-bit, I believe that requires both the OS (Windows 7) & the computer hardware to be capible. 64-bit Windows 7 will run at 32-bit t if all the computer hardware was made 32-bit. And 64-bit hardware will only use 32-bit if the Windows isn't 64-bit. If I'm wrong, someone can correct that.

Off-topic, If you're wondering... RAM is like a kitchen counter. If you don't have much counter space (low ram) you can only work on a small amount of food. Once the counter is full, you have to waste time moving the food you're done with so you can focus on new food. The more counter space you have, the less time you have to spend moving because there's room to just work around what's already there. More RAM = more countertop space.