Your current computer is probably a 32-bit PC, and you've never need to know it. But new the newer PCs are coming with an option for a 64-bit version of Windows, and Excel 2010 has a 64-bit version, so it's time to learn why these options are important to you.
The 64-bit measure refers to the "addressing space" of the computer. Imagine living on a street where the address numbers could only have one digit. How many houses could be on that street (assuming no house had a address of "0")? Obviously, only addresses 1 through 9 would be valid Now suppose that the house numbers could have 2 digits - there could be 99 houses, right? So you can see that the number of houses that could be "addressed"depends on the length of the address.
It's the same way for computers. If a computer only has 32 bits into which to place a memory address, it can only address 2 gigabytes of memory (plus another gigabyte by doing some tricks). And that's how most recent PCs work - they can only address 2 gigabytes of memory. This is irrespective of the amount of memory that can physically be put into the computer.
Newer PCs (those made in 2010 and later) have 64-bit addressing spaces. The amount of memory that they can address can't physically fit into any present-day PCs, but most PCs can now accommodate 8 gigabytes of memory. Running the 64-bit version of Windows 7 on these PCs allows use of all of this memory. This obviously quadruples the amount of memory that the user can utilize for Internet Explorer, multiple windows of Outlook, Excel and other applications.
But that's not the end of the issue, though. While Windows itself may be able to handle 8 gigs of memory, the 32-bit versions of application programs (such as Excel) can only handle 2 gigabytes. While that's an advantage over running the 32-bit version of Windows (because otherwise that 2 gigs would need to be shared with everything else that's running), it still limits the ability of Excel to utilize all of the memory that's available.
This problem is solved with the 64-bit version of Office 2010. With the 64-bit version of Excel running on the 64-bit version of Windows, Excel can utilize all of the physical memory of the computer that's not already being used by another program. This allows creating larger models that include significantly more data, and allowing those models to fit into memory, without needing to be swapped out to the disk which is extremely slow. This is a big deal to Excel users who manage large models, and even to users of smaller models that contain links to other models. And when combined with the new PowerPivot capabilities (to be described in a future blog posting) they allow manipulating massive amounts of data in Excel without significant slowdowns.
There are occasional issues in using the 64-bit versions of Windows. Printer drivers must be 64-bit, which are occasionally a problem for older printers. Some USB devices may not work with the 64-bit version, although even the Singletrack Analytics USB Missile Launcher (Google it to find out...) has a 64 bit driver.
If you're buying a new PC, you definitely should look at the 64-bit versions of Windows and Office. You'll really enjoy the speed of having a lot of memory.