Microsoft says a minimum of 2 GB is needed for Win 7 64. I would not be surprised to to see a lot of buffering issues and general lag using only 2GB. You should be able to run it but I would expect to see a performance hit. But if you had 2GB showing before... maybe no performance loss.
At $50-80 for a 4GB kit for DDR2 800MHz or 1066MHz ram, I think it is a little expensive or I'd recommend replacement - but it might be an option you can explore. See what you think using the 2GB, get a 4GB kit if you aren't happy.