I've heard that too but I don't think that's correct.
Here's my Take, 32bit XP has a 4GB memory "brick wall" meaning that it can only address a total of 4GB of memory. That includes any memory. Video ram system ram etc.
It's gonna subtract approx what it find in video ram from the system ram, in modern terms this means approx 3GB of the 4 GB installed in system ram is available for use. That being said, does it make a noticeable and or useable difference for you. Maybe maybe not.
I run 32bit XP on all the stuff I own and build to sell. Everything has 4GB of system ram in it and video ram of at least 512 sometimes more. Being modern machines, theyre fast. Fast as heck. Do I miss the 1GB of ram? nope.
Hardware Software and driver support can be an issue in the XP 64bit world, but it's gotten alot better. The good news is that it's easy to find out. Go to the mfg's website and see if there's a driver. Barring that google search for it and see what the users of said hardware/ software say.
It's your call and your dough. If you wanna run vista that's fine too.You've got a pretty big service pack released by MS in your favor plus all the units that have been sold since Vista's release are out there and working.
For my money I see vista as a twin of windows ME. An interim OS release that's just not up to snuff. When I beta tested it and ran it in my home environment it brought me nothing in benefits. Everything that came with it was a hassle. I spent hours looking up workarounds for stuff that should have been straightforward and simple.
It ran the hardware it was installed on just fine, altho it taxed the heck out of it just to show a desktop. Printer sharing and network file transfers sucked. The security stuff was a pain. The visual look was already outdated to me, looking like a bad copy of years old linux environments.
It sucked up a lot of resources to give me nothing and eventually went in the bin. I got it free and still didn't want it.
EDIT: wow that was long

. Short version. I work with PC's that use Vista and the various flavors of XP every day. I've never seen a PC that had vista that did anything better than and XP box. In the past that wasn't true comparing 98 to 2000, or 2000 to XP. The only time I've seen a comparison like this is in 98 to ME or 2000/XP to ME meaning ME was a turd compared to what else was out there.