Just realized that you are running 32 bit Windows. One thing I can tell you is that your video card (8500 GT) has its own memory on it and uses/takes nothing from system ram. This should help explain 4 GB on 32 bit, Coding Horror: Dude, Where's My 4 Gigabytes of RAM?
In your case right off the bat you are going to lose 512 MB due to the video card, with the rest of the hardware taking the rest. There is a god possibility that with changing the battery it caused the bios to reset, and enable/re-enable a setting or two for something on the motherboard that was disabled and hence the decrease in available ram.
By chance is the onboard video still enabled even with the video card? If so see if there is the option to disable it, and if not and you haven't already set the memory for it as low as it will go.