This happened to me when I was using an XFX 8600GT DDR3...... ONLY happened on X48. if I moved to X58, problem went away.
Today, it happened again with a different card. Same problem... this time I took a video.
Problem: 8600GT DDR2 comes up as a GTS. Device ID changes Driver choice makes no difference.
I'm posting this video so when I post results, no-one thinks im cheating or photoshopping