Couple of possibilities. If the RAM modules are from different manufacturers, rearranging them could have affected how they're "seen" by the BIOS. You could check for bad modules by installing just one stick. Usable RAM should be around 1-1.5GB depending on how much memory is set aside for graphics. Check each module individually. I'm not familiar with how many slots your Dell mobo has. Assuming 4, and you're using only 3, Dell may have a specific requirement to use slots 1, 2 & 3; or 1, 3 & 4; or 2, 3 & 4 etc for optimum RAM performance.
Another thought. Did you happen to change out or bump the CPU? A bent pin can also cause erratic RAM behavior.
If your BIOS was changed (sometimes by malware, sometimes by design) something called Memory Remapping could have changed. If Memory remapping options appear in the BIOS at all, make sure it is turned on.
You could also check msconfig. Click the Start orb, and type: msconfig. Open Boot tab > Advanced Options button. In that window, there is a setting for Maximum Memory. Make sure the checkbox is clear (not checked), then reboot your computer.
I'm running out of ideas.