Just some thoughts for Win Users.

Question

I am not an expert but just a regular forum brouser.
Is your system mother board is designed for Win XP? Yes,
then that is what you have got max user RAM 3GB. Your system board may accept two 2GB sticks but the system will only use a max of 3GB. Do not overclock your processor or activate the /3GB option, will not help you on PS or LR but may only cause problems.
You only get what you pay for no more, no less.
Win Vista same advice use what the system is designed for.
Win 7, make sure your motherboard is designed for the system and go for the full install and not the upgrade. If not be prepared to trouble shoot to get the performance optimized.
Do not expect Lightroom to sort out you OS problems.

Answer 1

Lots of good replies here; this is typically an OS issue, not an architecture issue. As long as the motherboard and CPU combo supports the (typically -- few of us run the Intel 64-bit server architecture) "amd64" architecture the 64-bit versions of the OS should work fine to see a "flat" (well, flat enough for Windows, which is rather lumpy in its approach to memory) 64-bit memory space.
With two caveats:
1. The OS still determines how much memory you see, and Microsoft has tweaked different versions and releases to see varying amounts of physical memory. Some see this as a defect.  See here: http://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx
2. Not all 64-bit operating systems are "64-bit clean." As far as I know, Windows 7 is the only 64-bit clean OS Microsoft has released. That is, the various pieces that make up an OS (kernel, services, driver, &etc.) are all 64-bit, with some of those pieces designed to handle 32-bit addressing if they have to.
(1) may affect user applications like Lr. (2) will almost certainly not, but it will affect overall system performance.
Lr does not require 64-bit to work (I assume it will in a future release -- the writing is on the wall) but the right architecture will allow you to run applications with larger address spaces, which means more resources and more efficient use of those resources.
Ironically, some folks will note that 64-bit has meant some things have gotten much larger. The process stack, threads and other internals have grown much larger with 64-bit platforms. This can sometimes be seen as a larger application footprint and even, in some cases, slower access to some system resources for 64-bit applications. These are extreme cases, however, and the best thing to do is run modest hardware that puts the emphasis on generally efficient processor and memory architecture and an OS that can take advantage of the physical memory you provide.
The point of the OP is well taken, however. Your "64-bit" OS as installed on your hardware may not, in fact, be able to see anything more than 2-4Gb of your physical memory. Older releases and OEM installs are infamous for setting things up so that all you end up with is a moderately more expensive and slow 32-bit platform.
Buy a real copy of the OS and install it yourself with none of the taskbar garbage OEMs love to give you.  Make sure you can disable real-time anti-virus protection, or remove it altogether (practice safe computing and you won't need it.) If you purchased from an OEM like Dell, demand your OS media (the are supposed to sell you installation media with pre-installed systems) and install it yourself (or get your local Windows enthusiast to do it for you.) Take regular backups -- actually, take two regular backups; hard drive "bathtub curves" are getting shorter by the year, while the cost per Gb shrink monthly.
Lr is a demanding app, but nowhere as demanding as many. It will use as much of the physical memory the OS can offer under the right circumstances, but the OS can only ration so much to various processes until something gives. Just because Lr is lagging on your system does not make it the problem! An OS is an ecosystem, and the kernel does its best to ration out resources to running processes.  Hungry foreground processes will just suffer more obviously from strained resources.