64-Bit Compatible CPUs, and the affect on 64-bit VS 32-bit OSes?

Hey guys,

I recently was given a slew of machines. They're all slightly older Dell Machines. Everything ranging from a Core 2 Duo 3ghz machine all the way to a 2.8 Ghz P4-HT. I'm probably going to upgrade some of the machines and give them to family members, and the rest I'll probably clean up and donate to underprivledged college kids that my sister-in-law works with.

So here's my question. All of the machines that I got, at the very least, support 64-bit. Some of them are dual cores, some with hyperthreading. I have a bunch of OEM Windows XP, Vista Business, and a Windows 7 disk from Dell. Since all of the machines that I have already have Windows OEM Keys on them, I will just use the OEM disks and use those keys so that I'm legitimate.

My question is, is there a huge difference between running a 32-bit OS on a 64-bit capable machine, or running a full 64-bit OS? Is it more a function of the OS to "enabled" that part of the CPU, or will a dual-core / 64-bit processor only run at say, "half speed" when run with a 32-bit OS?

I know that my Dell M1530 laptop orignially came loaded with Windows Vista 32-Bit. I purchased an OEM copy of Windows 7 64-Bit Pro on NewEgg and loaded it. The machine immediately went faster. I also upgraded my wife's 3.4ghz P4-HT with EWT64 from Windows XP Professional, to another copy of Windows 7 64 that I got on NewEgg also, and the speed difference was INSANE... totally night and day.

So my question is, was this other factors, or am I really, really hurting the potential of a computer by installing only a 32-bit OS on a machine, rather than putting the full 64-bit version on it? All the machines came with 32-bit OS keys... so if I do put a 64-bit OS on it, I'll have to buy them, I'm just wondering if it's worth it.

The processors in particular are:

- 3.0ghz Pentium Core 2 Duo vPro
- 3.4ghz Pentium D (two of these)
- 3.4ghz Pentium 4-HT (two of these)
- 2.8ghz Pentium 4-HT (two of these)

Thanks guys!

Todd