I had the 9100 and I sold it and bought an 8600. The 9100 was too bulky and heavy for me, and the power adapter is literally the size of a brick.
I absolutely love my 8600. I highly, highly recommend you go with the 8600. Dont buy extra memory through Dell as its more expensive. Get it through a differenct source like Newegg. 1 512mb stick of ram through newegg should be around $90... I dont know how much it cost to upgrade through dell but I bet it was around $75+.