There's more FUD and misinformation about this topic than just about anything else in IT currently. Let me try and clarify a few misconceptions.
It's correct that a 32-bit OS can only access 4Gb of RAM. Due to technical reasons (which I can explain if you really want but isn't wholly necessary to understand this), Windows has a 'blind spot' at the top end of the 4Gb address space meaning you lose about half a gig.
Now. Various forums will tell you that you "need" a 64-bit OS in order to use this missing RAM. Usually with lots of ZOMGs and exclamation marks for good measure. Technically this is true. But.
Moving from 32-bit to 64-bit will indeed allow you to access the "missing" memory. However, a 64-bit OS by definition *requires* more memory. You're doubling the width of memory addresses without adding more RAM; think about this for a second, if you've got a finite amount of something and you suddenly make it twice as wide, it's going to be half as long. You can't get something from nothing.
So, whilst it's correct that you need a 64-bit OS to access more than ~3.5Gb in a Windows system, all other things being equal it's a complete fallacy that this automatically is a good idea.
In practice, a good chunk of Windows x64 is still 32-bit, so you're not really doubling your memory requirements. However, you are increasing them. This will at least offset any theoretical gains you might see from the reclaimed half gig.
With a 64-bit OS, 64-bit drivers are *required* for all your hardware and, whilst the situation is better than it used to be, these aren't always available.
Under Windows x64, you can run 32-bit and 64-bit applications. The bulk of your software will still be 32-bit. 64-bit versions are thin on the ground except in certain fields (eg, video editing, CAD, SQL servers etc) but the tradeoff here is that this software requires more memory (as discussed). 32-bit apps are still restricted by their architecture; if you were to fit a hundred Gig of RAM in there, the 32-bit apps would still only see a 2Gb application address space (though you could run a hell of a lot of them concurrently). Also, 32-bit apps run under emulation (well, sort of, but that's the easiest way of explaining), so you can get a performance hit here too.
Finally, for 32-bit Windows, the actual real-world difference in performance between a 3.5Gb system and a 4Gb system (if such a thing were actually possible) will be as close to zero as makes no odds. You'd have to cane it pretty hard to use up all that RAM.
In short, it's a lot of work and effort to fix what is, for all practical purposes, a cosmetic problem. Don't worry about it. Anyone who tells you different fundementally doesn't understand Windows memory management.