My wife and I made the trip to DC through Gettysburg in early July of '05. We spent but one day on the field, but it was a good day. We took no formal tour. I just lead her up and down the line the Union held on day 2 and 3 of the battle. We didn't make it to McPherson's Ridge--or any of the day one sites for that matter--but standing on Cemetery hill looking over the field of Pickett's charge is a powerful moment. We walked all the way to Little Round Top and back, taking special note of the positions of the 1st Minnesota (the Virginia battle flag captured by this regiment on day three is still safe in St. Paul and well preserved--if they want it back they can come and get it
).
The weather was good (low 80's) and other tourists were more sparse than I would have expected. I saw no ghosts.
The next day we walked the national mall, moved on to Arlington (taking refuge in Lee's house during a thunderstorm), and concluded at the Marine memorial where the drill team performed (it was worth the $40 Virginia parking ticket). Two days of non-stop walking left our legs sore for a couple days.
My advice: Take your time and don't try to see too much. We were too ambitious and missed some things. I guess we'll have to go back.