Win 7- 32 bit vs 64 confusion

Instead of upgrading my 2 year computer, I thinking about putting the money to a new system instead. I have Photoshop CS3 and would be upgrading to CS4 to take advantage of the 8 gig of memory in the new system. No apparent problem except for one plug in that doesn't support 64 bit... then what? Also, I have variety of printers, scanners, a monitor calibrator that mostly have Win 7 drivers, but apparently not always 64 bit... then what? I also have a variety of software for music, video, panoramics, HDR, that usually have Win 7 upgrades but may or may not support 64 bit... then what? Also, what about external HDs, card readers, and MP3 players (non Ipods). Is there a switch mode from 64 to 32 when required or what happens? Is 64 almost like a different OS in regards to software and hardware? Confused. Thanks for advice.