Gas was already on the upswing when you bought your X5 a couple of months back. What were your reasons back then for purchasing it? Have those reasons changed?

If it were me, if the 325i is in good running condition, I'd use that as the daily driver until it croaks and leave the X5 for weekend use. Unless you have sentimental value attached to your '92 325i, it's not going to have any real appreciable value over a bone stock 325i because of its age.

As nice as your wife's Lexus RX330 might be, it's boring to drive compared to an X5, especially on a interstate trip.