64 bit OS's are very good for memory intensive programs, PhotoShop, CAD, Video Editing, the downside is sometimes you can't get 64 bit drivers for hardware, 32 bit is good for everyday users without heavy application demands, so what are you doing with your computer?
The main advantage as stated above is you break the 4 gig of addressable memory space barrier, that is about it. So, if you feel you need more than 4 gig of memory, you need a 64 bit OS. If what you do works fine with 4 gig or less, there is no reason to move to 64 bit.
There is no problem with 64 bit OS drivers or other issues with modern programs and hardware. Most issues are the fact that some programs simply won't work under the Vista or Win7 kernel, which is what you will be doing if you move to a 64 bit OS. If the program has issues with Vista or Win 7 64 bit, is is likely going to have the same issues with the 32 bit version. You can solve that by getting the Ultimate Version, which has XP compatibility mode. 64 bit drivers, you only need to worry about old hardware, like if you have a 10 year old scanner or printer, you might have an issue. But Microsoft has a program you download and run to let you know of any issues with your system before you make the change.
As Jitpublisher pointed out -- the most noticeable difference between 32-bit and 64-bit is how much memory can be addressed. To learn more about the differences to 32-bit vs. 64-bit architecture, please go here: http://bit.ly/oj1fV
jitpublisher was right about the 4 gb barrier. It is why 64 bit is becoming popular.
However I don't think he made clear that 64 bit does break compatibility with some older programs. And they don't have to be 10 years old. Nor are all the driver issues 10 year old hardware. But if the exceeding 4 gb of ram is your goal, it is the best option.
All above comments are good.
(1) as previos posts have stated if you plan on going above 4 Gigs Ram, You need 64 Bit. A good example is if you are going with a MB that requires 3 memory modules to run in triple channel mode (x58 boards) you options are 3 gigs ( 3 x 1), or 6 gigs (6 x 1, or 3 x2 Most popular). H55/57 MB use DDR3 in daul channel so that you could install 2 x 2 gig modules.
(2) The 32 bit will allow better compatability with Old programs. I did have a driver issue with 64 bit for a PERFECTLY good scanner, NOT 64 Bit proble, it is that Cannon refuses to publish a 64 bit driver ( you can do a work arround using the driver from a newer Model and contend with error message). Cannon is not the only one that is tring to generate revinue by making you buy new hardware vs suppling a driver.
(3) The program compatability issue is tied to older programs that may have 16 bit code embedded- Not compatable with 64 Bit.
(4) How many programs do you have that are 64 Bit programs. If memory =< 4 gigs and you do not plan on upgrading programs to 64 bit I'd probaly go with 32 bit. On the other hand a 64 bit machine will alow you to gradually upgrade programs to 64 bit ( better performance - Not sure how notable on daly tasks. And More viable in future.
Me - I have 2 laptops, 3 Desktops. All have 4 gigs ram and all except my I5-750 system are 32 bit machines. On the one laptop with 32 bit Win 7 I have been able to install some very old programs and with a little tinkering - they work. One program was written for win 3.11 and interfaces with an O'scope via serial port. On the other side (64 Bit), I have Basic professional a DOS 6 Program and it runs fine on both 32 & 64 Bit Windows 7 (only problem is with code that intefaces with hardware and programs I wrote that used embedded Printer codes of course don't work.
This question is asked frequently and my answer is always if you have 4 BG of RAM, go for it. If not, 32-bit is the way to go. Whichever you choose, make sure your current programs are compatible before installing. Also, if you have 32-bit now and decide to go to 64-bit, you will need to do a custom installation to do so. Here is one additional resource that compares 32-bit and 64-bit - http://windows.microsoft.com/en-us/windows7/32-bit-and-...