You might've skipped the Vista train, thinking it was like Under Siege 2, minus Steve Seagal. Or not. Either way, you're probably gonna jump onboard Windows 7. When you do, it's time to go 64-bit.
Who Should Go 64-bit?
Basically, anyone geeky enough to read this. If you have an Intel Core 2 Duo or newer processor, you've got a 64-bit CPU, and you should install the 64-bit version of Windows 7 to play with. (Here's how. You've got like 5 days left, BTW.) Microsoft itself is pimping 64-bit over 32-bit now and notebook makers have already started pushing 64-bit Windows Vista over 32-bit. Mac OS 10.6 Snow Leopard will be 64-bit down to its kernel, so you won't be alone by any means. 64-bit is going mainstream.
Why Should I?
We explained what's so awesome about 64-bit in detail a couple months ago, but to recap in a single word: Memory. With 32-bit Windows, you're stuck at 4GB of RAM, and even then, you're only using about 3.3GB of it, give or take. With 64-bit, 4GB of RAM is the new minimum standard, and with 4GB, you can run tons of applications with zero slowdown. Windows 7 (and Vista for that matter) runs so beautifully with 4GB of RAM you'll wonder how you ever did with less. It makes your system more futureproof too, so you can take your system to 8GB, 32GB or even a terabyte, before too long.
Who Shouldn't Go 64-Bit?
If you're not planning on going to 4GB of RAM anytime soon, you might wanna hold back, since you need 4GB of RAM to take full advantage of 64-bit's memory management. That said, RAM is so disgustingly cheap right now, and has such an intense bang-to-buck ratio, you should definitely upgrade to 4GB if you haven't already. Anyone who runs specialized or older gear (see below) should probably not jump into 64-bit.
It's true that 64-bit Windows used to be dicey on the driver and compatibility front, but from Vista onward, it's typically nothing you have to worry about. Most new hardware has 64-bit drivers, and even though most applications aren't 64-bit native yet, 32-bit ones usually run just fine.
Still, the biggest issue is hardware. If a gadget doesn't have 64-bit drivers, it won't work with your 64-bit OS, since 32-bit drivers aren't supported. Most non-crusty gadgets should be okay. (Seriously, I've run 64-bit Vista for a year, and now Windows 7, and everything I've tested for Giz plugs in just fine.) But if you run legacy goods, it might be kinda sticky, and you should still double check your gear just to be safe.
There are a few software issues to look out for, too. Google's Chrome, for instance, doesn't play nice with Windows 7 64-bit for some people (like me). Adobe Flash doesn't run in 64-bit browsers, but that's not really a problem—you can just run the regular 32-bit browser instead. iTunes had problems with 64-bit versions of Windows in the past, too (granted, Apple's not the most fastidious Windows app developer out there). Most of these issues have been or will be resolved, but if you use specialized mission-critical software, definitely read up on its 64-bit compatibility.
Really, Go 64-Bit
The caveat section looks longer than the "DO IT" section, but really, you'll probably be just fine running 64-bit. A ton of other people will be 64-bit with this generation of OSes/hardware too, so you won't be alone. The benefits of oodles of RAM, given all the crap you're running simultaneously, are just too good to pass up, especially once more apps are 64-bit native. Besides, the more people that jump on the 64-bit Express, the faster developers will transition their apps to 64-bit, and any bumps in the road will be smoothed out. So don't just do it for yourself, do it for everyone.