Assuming that the Pats don't draft someone off the current radar, let's look at the "consensus" picks in the top 7 and how I think the Pats react if they are there at #7:
Matt Ryan
No way they draft him, but is an ideal choice for a trade-down. Strong demand for QBs and the market is not so hot. Ryan has ideal measurables, a strong-enough arm and top intangibles. He may not turn into an All-Pro, but it is unlikely he will be a bust. Trade down and get a #1 in 2009 for the best crop of LB in quite a while.
You'd have to trade down pretty far to get a #1 in 2009 for him.
Jake Long
Hard to pass on a top LT prospect. Fits from a need/scheme/money perspective. I wouldn't go in this direction, but wouldn't fault the Pats for doing it. The trade down possibilities would be tempting as I'm sure sure there would be a good number of teams in the low-to-mid 20's (the Pats drafting sweet spot) that would love a franchise LT.
Actually, I think the Pats would love to pick at 15 or 16, where the cost isn't all that much higher, and you can sign the player for six years instead of five.