I am in the same boat. I bought my Boxster new in 2001 and have decided to move over to a 2004 or 2005 996 turbo cab. I listed the car in early November and have yet to sell it. However, I am asking top dollar for it; $23,000. If it doesn't sell for close to that I will keep it and still get the 996tt cab or coupe. $23,000 is not going to break me and I would rather keep it than let it go for less than I think it is worth.
If you can afford to keep it, do. I don't think you will regret it.
http://986forum.com/forums/boxsters-sale-wanted/49380-fs-2001-porsche-boxster-s-23-200-miles-original-owner-loaded.html