I think we should get Briggs, and I think we CAN get Briggs for a first round draft pick (no third).
We need him, not for this year, but for the future.
In 2008, Colvin will be in the final year of his contract and a potential cut or trade*. Bruschi will probably be retired. If we get Seau for this year (as I expect) it will probably be his last (he just wanted a ring). Vrabel may still have something in the tank, but not much. In short, our entire LB corps is being disassembled, and we need replacements.
The biggest stumbling block for me is the potential for Briggs to demand more than he is worth. But if the Chicago situation gets ugly enough, Briggs will have the incentive to take a below market deal (just to get the hell out of there).
*I'd keep Colvin in 2008 if his 2007 matches his 2006. I'm just saying that its close enough that the Patriots will have to examine it, and a poor 2007 could easily change their minds.