I don't know how much you can lose (as that depends on your size, etc). But I can assure you that if you follow Sole Source to the letter, you will lose quite a bit around the stomach. I don't really understand how it works but in one week I had lost 1"!!!
Is the dress too small? If so by how much? Is it at the bust or at the waist or both?
If you fell that it's tight, I can recommend M&S shapewear, especially the ones that go from mid-thighs up to under your bust. Not very sexy and not very comfortable either (they DO squeeze you!) but it works wonders (for me anyway). xxx