I didn't read through all the replies, but I have one comment to add:

There is a bug in Vista (not sure if it's unique to x64 or not) that makes it blue screen if you try to use more than 2GB of RAM while installing or using the OS. There is a patch for this bug through Windows Update, so once the patch is installed, you can put the extra RAM back in. I don't recall the exact name of the patch, but if you search Microsoft's support site, you will find references to the bug. I believe it was related to motherboards with Nvidia chipsets.

Bottom line though - Start with 2GB RAM (or less), Install Vista, update all patches, install extra RAM.