Eliminate unnecessary programs running in the background
Check that your video and audio drivers are up to date
Memory can indeed be an issue - the game's use of your ATI memory, as Treebeard explained in the ATI guide.
This has also helped a LOT of people with crash/rgl errors: (8th time posted links in this thread)
http://forums.taleworlds.com/index.php/topic,131274.0.html
If you have an ATI card, read Treebeard's guide to dealing with ATI problems, stickied on first page.
If you have not tried any of the above, I'd suggest trying some of these solutions. Many people posting in this thread have had similar issues which were corrected by one or a combination of the above. Don't dismiss a solution just because you think it has no relation to your problem - it may well have, but you will never find out unless you try a few.