I'm sorry to hear it, but chances are very good that your dog will be fine. The vet will most likely remove the tumor, and try to get good clean margins all the way around, then send the tumor off to be graded. If she got clean margins and it's in an early stage, most vets believe that's enough treatment. If she couldn't get good margins, and the tumor is slightly more advanced, then you have a number of options.
To be honest, I'm kind of surprised that they referred you to an oncologist before doing the surgery to remove the mast cell tumor. Getting a second opinion is never a bad idea, and it can reassure you a lot.
Here's a good overview of mast cell tumors:
http://www.peteducation.com/article.cfm ... 38&aid=461
My elderly Tiva had a mast cell tumor in her tail last spring. The vet removed it, but the pathology report came back at "grade II", so simple removal was not enough. We consulted over the phone with the state vet school's veterinary oncologist, and she strongly recommended amputation over radiation or chemo (the tail location made this an easy choice). So Tiva had her tail amputated, and she recovered extremely quickly (ie, in a day or two, she was raring to go, and in 2 weeks, the incisions were healed up nicely).Almost a year later, she's still completely fine. We watch for new tumors, but so far, so good. This is a very common outcome, so don't panic!
The fact that your dog's tumor is on a leg is good news--mast cell tumors on limbs are much less likely to have spread than those in the core of the body.
Keep us informed, get a second opinion if you want to, and don't panic. Personally, I would see another vet more quickly, since the first line of action for mast cell tumors is remove the tumor, then grading it. Only after surgical removal and grading do you move on to radiation or chemo if necessary. I don't do waiting very well, so I'd just try to see another vet, and schedule the surgery asap.