I recently went through all this.
I had been running XP32 for years, because there were always issues with first Vista, then 64 bit. I still run into 64 bit issues, but they are getting fewer and fewer as time goes by and now, finally, they were few enough that I could switch.
I would periodically do a dual boot and test everything out for evaluation, but it wasn't until this spring that I even went to Vista, and just 32.
I recently installed Win7 64, and tried that out - for the most part no problems, however the speed was identical for me, and I don't care for the look. For me, most of it's changes are simply cosmetic, and I don't use the features anyway. I know how to tweak Vista for optimization, and doing back to back tests and benchmarkings confirmed that both systems were awesome performance wise. I don't plan on going to Win 7 anytime soon, since it has no new features I need/want. Out of the box though there is no comparison - Win 7 whips Vista's ass - you MUST spend time configuring Vista to get it where it should have been to begin with. So I definitely recommend Win 7 to anyone because it's easy and works - I just don't need to spend the money on it over what I have.
Anyway, I switched to Vista 64, put in 8 GB and am running a 3.4GHz Intel quad. Everything freaking flies - the bottleneck by FAR is the disc system, and I use Raid 0 arrays for speed. I'd really love to plug in some SSDs, dedicate some for the swap discs...maybe later. My biggest improvement was running Lightroom and PS at the same time, with a bunch of images open. On 3GB I would max out with just a few files and sometimes need to reopen Photoshop to get speed back. Now, SO much faster with more going on!
64 bit is NOT without problems though. The OS itself is great, the 64 bit apps are great - but it's the little stuff that's lacking.
Some flash apps don't work in Win7 64, like AOL radio for example. They so far all work fine in Vista though.
There is no 64 bit Jet DB, so nothing working with Access databases can work. So not only no MS Access, but I have things like my Cigar Registry that uses a Jet library - no go. MS is rumored to be releasing a new DB app using SQL to work with the next Office and dropping Access - but plenty of apps use Jet.
Lots of NON-current version apps don't work, or will even install in 32-bit mode. Quickbooks for example - I had to get the latest version to even install it. Not happy about that.
Lots of NON-current version Photoshop plugins either don't work in Photoshop 64, requiring you to launch Photoshop 32 defeating the purpose, or don't install on a 64 bit OS. Some have beta versions, most require you to purcahse new versions. I had to upgrade Noise Ninja, my old PK Shapener didn't work, and none of the Nik tools work in PS 64.
SO it really depends on what you need to do, if it will work for you in the end. If you require use of plugins that don't yet have 64 versions, there is no point since you can't run PS64 to take advantage of the 64 bit differences like extra ram.
Luckily what I do is run both versions of photoshop, changing when I need to since I only need plugins for some parts of my work and can run 64 bit for the others where I can really use the extra ram like large prints. I only use the filters on my wedding shots, and they don't get large.
