Yes, both the processor and OS must be 64-bit to enable more than 4GB of RAM.
Pretty much any processor made within the past 2 years supports 64-bit addressing.
Your mileage may vary when using the 32-bit flavor of Vista (or XP or Server 2003 for that matter), but my take on it is 95% of the people out there won’t be able to see all 4GB of RAM unless they go to the 64-bit version of Vista… there’s just too many caveats and sporadic motherboard support.
It’s really worth the upgrade to 64-bit in my opinion. Compatibility is starting to be very good, and once you have more than 4GB then you can enable SuperFetch!
I have 8GB of DDR2-800 in my machine (running 64-bit Vista Ultimate), and SuperFetch actually speeds things up considerably when you have this much RAM.
I have a few servers that won’t see more than 3GB, no matter what flavor of Windows Server you throw on it… 64-bit or otherwise… and it’s meant to be “professional” hardware. They touted how it can run a 64-bit OS, but this is rather pointless because the main reason to run a 64-bit OS is for the access to additional memory.
I forgot about the Celeron chips… some of those are crippled… I bought a cheap 1.8Ghz Celeron and it doesn’t even support on-die temps! If you have a Celeron I wouldn’t even try to enable 4GB of RAM cuz who knows what Intel has done to them to cripple them?