Lackey for $17/year seems to be appropriate value given his recent track record and the few options in the market but the Sox still need a cog to replace Bay.
Lackey will help just as much as a good hitter over the course of the regular season but once you get to the postseason, having that much talent, Buchholz and Daisuke, in the 4th and 5th rotation slots seems a bit wasteful. IIRC, James' projections have them both as low-4 era pitchers this year. Especially if you're going with Kotchman and a Hermida platoon to replace Bay. Just doesn't make sense.
Taking a peak at the Sox' payroll going into next year. If they commit to all the remaining players on the books for 09, I'm estimating they will be at $130-135 mil (taking Lowell trade into account). Add Lackey (let's say the contract is spread evenly) and that goes to $147-$152 a year. That gives you about $20 mil to work with before you hit the luxury tax threshold. The Sox hit that threshold in 2004 so they would be penalized 30% for every dollar spent over $170 mil.
I'd love to see them sign Holliday. He is the jewel of this year's free agent class but it will take $20 mil/yr to get him. The Sox hiking up payroll $50 mil in a single offseason (from $121-170) would be unprecedented for the Sox, even in the Duquette era. It seems more likely that the team is preparing to package Buchholz for a young hitter. It seems counter-intuitive that the Sox would trade a young pitcher but given they are reportedly trying to re-sign Beckett, Buchholz' role would be less important with three "aces" signed to long-term deals and Daisuke on the books through 2012.