All bios are different. You should be able to consult the hard copy or online copy of your motherboard manual to setup the RAID again. The steps aren't too difficult and usually safe. Professional advice is to back up the drives before doing so, in case there's data you're not willing to slightly risk.
There are a few different methods to update the bios. They are outlined in the manual for your motherboard at the previous link I gave. Open the .pdf and then search for "update bios", without the quotes.
Not really sure. Here's something you can try though.
Type msconfig in the start menu and then hit enter. Go to the boot tab and then click on advanced options.
See if the box is checked or not for "maximum memory". If it is checked and/or has anything other then your fully installed memory, try removing the check from the box OR setting the
value to be 12288.
Hit ok 2 times to save then reboot and check if it made a difference.
To find out how your memory is allocated go to task manager(performance tab) and click on resource monitor. Then click on memory tab. At the bottom is a graph that shows how memory is allocated. Post a screen shot.