The 64 bit version isn't all that great. to kinda explain it it's able to run 64 bit applications like ash said cad software ect. it can also run 32 bit programs as well the problem is it goes one way or the other so if i'm understanding the way it was explained to me is say your trying to run a 32 bit application but you drivers and such have 64bit drivers it will have some issues running the program or may not run it at all the 32 bit version is thae way i went because there just isn't many programs out there that make use of the 64 version. So mainly it depends on what you use your systm for.My system is what i would consider a gaming rig and i won't run the 64bit version. This is my setup.
1.Msi Kan ne04 platinum sli
Mobo
2.amd athalon 4800x2 gual core cpu
3. 3 gig corsair platnium ram.
4. two evga 7900gt co Vga cards in sli
5. Four 250gb seagate hard drives (way overkill)
With all the stuff i got wrapped up in my system i chose not to have the headachs of running 64bit version save some money get some better toys for your pc