IMHO - I'd sell your X5 yourself and then go buy what you decide on. Dealers goal is to make money on your trade and then on the new.
Keep the replacement purchase simple - with no trade you will have a better place to bargain from. Also, you need to get a quote from a dealer or use a source (like edmunds.com) as to what you think you can do on trade versus selling it private party. If your sales tax diff isn't that great then there's your answer. If you can get someone that has access to manheim dealer auction that will give you an idea of what your car is going to bring on trade.
Example-
I got quoted $14,000 for trade in value on my 2004 - I sold it privately about 5 or 6 weeks ago for $22,000. I'd take the $8K in cash over the smaller sales tax savings.
