Need opinions on OS software

So I found out, which surprised me, that my daily use computer Dell Vostro is a 64-bit. Installed I have Windows Vista Home Premium 32-bit, and I have another Windows Home Premium 64-bit.. can I just re-install this 64-bit, idk if it ever asked me for a serial since its Dell OEM OS. Or am I going to run into issues with activation? The other Dell now has Windows 7 on it, so this Vista disc is just sitting around. And the benefits of 64-bit persuaded my decision.. If specs are needed, here they are

Intel Pentium Dual 2.2GHz
2GB DRAM

IIRC you cant upgrade from a 32bit OS to a 64bit OS or vise versa.

All my rigs run Win7 save for an old laptop that runs Xubuntu, so id say go to Win7.
As long as you have a valid Win7 CD Key for each computer it should install just fine.

I don't know about vista, but I have been able to activate a 64 bit installation of windows 7 with a 32 bit key, no problems. Make a backup of your current system and give it a try. Also if you have access to a .edu e-mail address you can get win 7 pro, 32 or 64 bit for $64.95 from microsoft student discount (google search it)