If it was me and I was expecting to fly more next year, I would make the effort to hit silver so as to at least get the shot at upgrades and the mileage bonuses. Relative to the WC membership, it all depends on how much you value it and how much you intend to use it in the future. If you will be traveling more in the future, it seems reasonable that the WC membership may be worthwhile. It would make sense to get it while still Gold, though.

From a quick check, it looks to me like a PHL-DTW-SDF-DTW-PHL trip is about the cheapest same-day return available at $248.50 all-in. You'd have to spend at least 3 hours at SDF to get that fare, although a plus would be that an extra hour in SDF could allow you to fly entirely on mainline (LEX guarantees Airlink).

EDIT: An additional note; it appears you could actually fly one-way PHL-JFK on Delta to make the 500 EQM on 12/29/08 for $198, could potentially be cheaper than some NWA trips if you can get back to PHL through other modes.