It freaks my wife out how in my mind I can take an object and turn it, rotate it, and visualize every aspect of it in 3D . . . but I can't draw it to save my butt.
She can draw pretty well, but can't visualize what the hidden side would look like if you turned it around.
I'm more like you then Web, I'm pretty good at visualizing something, but can't draw worth a darn. I sure wish I could. One side of my family is amazingly artistic. My grandfather used to have a huge 8 foot mirror in his living room and every christmas he'd paint it up with a holiday scene.
It was a cool thing every year to get down there on Christmas Eve and see what he'd come up with.
As he got older he started sharing the tradition with his sons, and many of them were equally as good.
When he passed away six years ago they had one of the scenes he had painted etched onto his tombstone. It was a really cool snow covered hill with a little shack on it and tree's and stuff. Very neat.