Becky,
I remember hearing in college about choosing the sex of the baby by your ovulation schedule. Looking back at babies 1, 2 and 3, it was right on. We weren't trying for a specific gender, it just worked out that way. With #4, we really were hoping for another girl, so we intentionally looked at the calendar to time ovulation when planning to conceive and we got a girl. That is four for four.
Just remember that your influence has a limit. Even if you paid $6K to have your husband's "sperm spun" there is an 8% chance you would still have another boy. All you can do is put the odds in your favor.
If it was me, I would sit down with my husband and decide what you REALLY want. If the most important thing is getting a girl and you don't want more than 3 kids, there are a lot of precious babies in China who need a home. If you are willing to go to more like 4-5 kids or would feel complete with 3 boys, you may want to try for the girl and see what happens.
I think you just have to go with your gut on this one. After 3 kids, we knew we were missing a little girl who was supposed to be in our family and we were going to get her one way or another, even if it meant adoption after 4+ kids.
Good luck,
S.