There's no reason to run 64 bit Vista on a machine with only 4 gigs of RAM.
The overhead of the 64 bit libraries plus the 32 bit WoW (Windows on Windows) libs take away any benefit of being able to address the full 4 gig of physical memory.
Other than the ability to address more memory, there's no other benefit to running 64 bit over 32. Even if you do have say, 8 gigs of memory, in order for an application to take advantage of that, it also needs to be a 64 bit executable. There are still very few of those. Photoshop is one of them, though. When you go into the performance settings on PS CS4 64 on Windows, you'll see that you can assign more than the 32 bit application memory address space.
Even though Vista 64 has vastly improved in terms of application and driver support over XP x64, it's still not as seamless as 32 bit. For example, ever since SP1 came out, I stopped being able to do divx transcoding and unless I use an unofficial hacked sound driver, I can't get 5.1 out of the sound card.
As for XP being "dead no matter how you look at it", that's patently untrue.
With the exception of Windows Me, there's never been a Microsoft OS less embraced by corporate IT than Vista. The reason is that MANY business applications still won't run properly on it. I don't mean biz apps like Office, I mean financial systems and other functional applications. Look on any corporate PC, and chances are you are going to find XP running. I wouldn't call it a dead product.
Also, in terms of performance, XP is still yet to be beaten. There have been reports of some positive movement in that regard with Win7, though. My own benchmarking has seen Win7 pwn Vista64, but still not approach the 3DMark scores I get under plain old XP 32 bit.
Another infamous benefit of XP over Vista is simple file copying. I read an MS whitepaper on why Vista file system transactions are so piss poor slow and the theory made sense, but still... I'd like for files to copy at maximum speed and in the minuscule chance that there's an unrecoverable error, I'd rather deal with whatever the caused the problem than have every copy move at a snail's pace.
In terms of gaming, the only advantage to Vista is DirectX 10 support. However, DX10 has been pretty much an unfulfilled promise. Sure environmental effects look prettier and water/smoke looks awesome, but that crunching sound you'll hear is your framerate hitting the floor.
Also, many game devs consciously choose to stick with DX9, just because it's leaner and higher performing.
Personally I run Vista 64 on my main PC desktop, which does have 8 gigs of ram (so I can run fully loaded VMware guest OS's of XP so my work stuff will run, since I'm a business application guy). It also dual boots to XP for some games, etc.
On my MBP, I run XP in Fusion as a guest OS for doing work stuff and a bootcamp of XP for gaming. It's only a lame laptop, so I don't want to give up a micron of computational power to Vista's overhead when all want to do is kill stuff.
John
Here's a Microsoft breakdown:
