The Nationals recently signed Jason Marquis for 2 years at 15 million. The Nationals are going to have to overpay to get decent free agents, so that's fine. Here's what would really worry me: He has a 94 wins and a .531 W-L%, which isn't bad, but a 4.48 ERA. In ML history, there have been over 300 pitchers with at least 94 career wins, and a .531 W-L%, and only 5 have had worse career ERA's than Marquis.
Most of these guys really started to flame out around age 30 or 31 (Marquis is currently 31). In 1999 (age 32) Charles Nagy went 17-11 (with a 4.95 ERA), and after that, he went 8-19 with an ERA of 7.50 over the next couple of years. A few years after being a Cy Young contender, Russ Ortiz went 0-8 at age 32, and has put up terrible numbers while barely staying in the league.
Burba was a bit of an exception, because he didn't become a regular starter until he was around 30. Marquis, like Sele, Ortiz and Nagy, has been starting since he was about 24. Oliver also lost it as a starter in his early 30's, but saved his career by become a good reliever in the last few years.
Marquis ended up last year on a bad note as well, losing 7 of his last 10 decisions. The Nationals probably aren't expecting that much out of him, just to be a professional starter who can thrown 200 innings and be slightly above .500. With low expectations, he has a decent chance at being what they expect in 2010, and it's only a 2 year contract, so it's not too bad. If Marquis is anything like Nagy, Ortiz and Sele (and he appears to be), it could be a very bumpy ride starting soon.