I need to change my CMOS battery and have read different views about needing to save my bios settings. I have read that the bios just resets to default and the only changes will be the date,time and boot sequences. These are easy to change back so, do I really need to take photos of each screen? Thanks in advance to the techs..

I just had the battery replace in my old XP computer. The bios resent to default and the only thing to set was the date and time. But wait to see what the techs say. I am not one! Good Luck!

That depends on how much tweaking you've done.
If all you've ever done is reset the date, time, and boot sequence, those will be enough. If you've added video, network, or sound cards and disabled onboard cards in the BIOS, you need to reset those after changing the battery. Look through the BIOS and see if there's something you might have changed. If you don't see anything, you should be fine. If there's something you're not sure of, write it down or take a picture. You'll probably find very little that needs to be changed from the default settings.

I recently upgraded my graphics card - from Nvidia 8600GT 512MB to Nvidia 640 2GB. Could this of caused my 'cmos checksum error' on start-up? My computer is 8 years old, never changed the battery and the clock is losing time (approx 10 mins in 24hrs)have presumed its the battery.

Changing the graphics card shouldn't cause a checksum error.
A checksum error can be caused by a failing battery.
Losing time is usually caused by a failing battery.

Adding a graphics card to use instead of the onboard graphics may require you to disable the onboard graphics in the BIOS. Many computers detect the new card and automatically disable the onboard graphics, so you don't have to do it manually, but it depends on the BIOS.

Thank you Jenny, will change the battery and see how it goes.