But what type of defense do they want to run - a 3-4 or 4-3? Of course you'd want them to run both, but it's been hard enough for BB to settle for LB's that fit his system and can make an impact in a 3-4. At this point, I'd say it would be easier for the Pats to continue to build around a 4-3 type defense and focus on drafting pass rushing DE's. Assuming McKenzie regains his form, he could be a great fit as an OLB in a 4-3.
Regarding AT, I think he's played his last season with the Pats. He flat out hasn't played well in a 4-3 or 3-4 for the Pats. In addition, although Guyton isn't the greatest at setting the edge, he's good in coverage and I like him as the other OLB in a 4-3 defense.
In the end, with a potential uncapped year in 2010, the Pats can very well shore up their pass rush by signing a veteran (Julius Peppers) and draft a couple in the 2010 draft. Assuming the Pats re-sign Wilfork and make a splash in FA, their defense can finally take it to the next level.