Windows 8 is still in beta, not recommended to replace windows 7 yet..
If you want to play with windows 8, I'd recommend a dual boot configuration (using a seperate HDD).
Disconnect HDDs, except the one you plan on putting win8 on (NOT on the win 7 HDD.
Instal the beta windows 8.
Reconnect other HDDs. Leave bios Boot prioity set to the HDD with win 7 on it.
use the Hot key that prings up the boot menu.
.. On my Gigabyte MB it's F12 and on my Asrock MB it is f11.
.. Select the HDD with window 8 on it.
.. Using this does NOT change Boot Priority.
.. If in Win 8 and want to bounce back to win7, just select restart and computer will reboot back into window 7.
So Your question is going from 32 bit to 64 bit and should you go straight to win 8 64 bit???
I would not go to windows 8 beta 64 bit for my main operating system, unless you can verify that you would not have a problem. Go with a dual boot system while verifing windows 8.
When windows 7 beta came out, I did try that using as a dual boot with vista. The last stage of the "beta" is the pre-release vertion. But I did not transistion to using windows 7 as primary operating system untill it came out. Pre-release versions do not always support every thing that a user needs (Varies for individuals), also may have a bug that can bit you in the A$$. The final version of windows, prior to release was pretty good.
Also (1) beta versions have a "life time" built in. (2) you can not use an Upgrade version of windows 8 to upgrade the beta version of windows 8.
You can do an 32 bit to 64 bit upgrade via licensing requirements. In other words if you have a 32 bit version of Windows you can buy and install an "upgrade" version of 64 bit Windows. But what happens is the new version will save your old 32 bit version back into a folder, and then do a completely new install of the 64 bit OS version. The reason is because the 32 bit kernel is not compatible with the 64 bit kernel for upgrading purposes. You will have to reinstall all your programs, it will be a fresh new installation.
Note that 32 bit programs run fine under a 64 bit OS. If you have problems with any programs running, it will be a conflict within the OS its self, it will not have anything to do with it being 32 program running under a 64 bit OS. If a program gives you an error message when you try to launch it, or it refuses to launch at all, it would very likely do the same thing if the new OS you just installed was the 32 bit version as well.
As mentioned though, Windows 8 is still beta, and people are reporting a lot of bugs and programs simply do not work with it.....yet. I am sure by the time the final release is available, there will be much more support and the bugs will be fixed.
It is HUGELY advisable that if you do want to run Windows 8, you do it as a secondary install for a dual boot and leave your old Windows installation active and usable for now, as you are going to have issues of some kind, we guarantee it.