Photoshop CS5 is a 32/64 bit application, so when you install it on your new system it will install both 32 and 64 bit versions by default. This will give you compatibility with your 32 bit plug-ins, as well as 64 bit capability to edit large documents.
You may want to avoid the upgrade to Windows 8 at this time. It's still immature and the drivers (e.g., for ATI video cards) leave something to be desired. Plus, in general Windows 8 doesn't offer any compelling new features over Windows 7 (and actually removes some very useful one) - unless you really want the silly full-screen Metro/Modern games they offer at their new App Store.
If you DO decide to install the Windows 8 upgrade, I recommend you take whatever steps are necessary to install the new system as a full, virgin installation, rather than trying to upgrade the Windows 7 system you already have installed - and by this I mean you should NOT take the "keep programs and settings" upgrade, but do a Custom installation and install Windows 8 as though on a blank disk. Operating systems installed as upgrades to prior versions virtually never work right.
In any case, after having run Photoshop on Windows 7 for a while, you'll want to DEactivate your Adobe software before making the transition (you can do this through the Help menu), then reactivate it after you're up on the new system. This keeps control of your activation/licensing in your hand.
I strongly suggest you read up on Windows 8 before deciding to upgrade. You'll find a lot of experts agree that it's not an improvement over Windows 7, which is hard to imagine because all prior versions - even Vista - have been improvements over their predecessors.