How can I upgrade from Vista 32-bit to Vista 64-bit?

Does anyone know if my product key for Vista 32 will also work for Vista 64 for an OEM copy?

Yes, the same product keys work for both 32 bit and 64 bit versions of Vista. Not sure why there's so much misinformation about this on the 'net. I know because I've done it!

As far as I know, I just need to get the 64 bit media.

Yes.

According to the Microsoft Site, you can "upgrade" your 32 bit version to 64 for free, minus the shipping cost of a 64bit version on DVD. I'm not 100% sure how it works, but I believe they'll re-register your key to be legit for 64bit Vista.

I've been looking to do the same for my Inspiron 1721. If you go this route, let me know how it works.

One slight twist to the 32/64-bit thing:

If your key code is for Vista with SP1 built-in, you have to use a disc with SP1 built-in, or activate after SP1 has been applied. Apparently the key code mechanism was updated with SP1.

This is probably where the misinformation comes from - if you had a 32-bit disc from a promotional event with SP1 built-in but you tried to use the key with a non-SP1 64-bit disc, it would not accept your key and/or would not activate. You would have to use a 64-bit disc with SP1 installed, or install without a key and then apply the key and do the activation after SP1 has been applied.

Also I think Vista Ultimate and maybe Business are the only ones with both 32-bit and 64-bit discs in the box - the other versions just have the one disc for the version listed on the box.

I used a 64-bit DVD I had, entered my OEM product key, and Windows activated with no problem.