If I were you, I'd leave my car at the closest Metro station and just take the orange or blue line to Stadium/Armory. I don't get down there often, but I'm guessing... Vienna/Springfield?
Just thought I'd edit a little more into this -- if you pick up the train at V/S, you'll be able to get on the first car, which is the one you want. And when you leave the ballpark, go PAST the mob waiting at the station entrance closest to RFK and enter at the DC General end.
Best of luck. Go Nats.
This advice needs to be reiterated: do not bother trying to enter RFK from the main gates
. Instead, when you get to the stadium, turn right, walk down the stairs (you pass through a chainlink fence) and walk to the entrance down there. It's only 70 feet away, and it's always an empty gate!