Oberlin does send students to PhDs frequently enough, and enough adcoms should be familiar with their program, should so you should be good on that end. The TA stuf will not help much for admissions, but might help get better funding. The RA stuff however helps with admissions though how much it helps depends on your role in the project.
Having multivar calc on your transcript, even as a course in progress when you apply is pretty much an absolute must for a top 20 finance PhD program, and without at least one of real analysis or differential equations you will be an extreme longshot. Admissions for top finance (and econ) PhDs have become much more competitive especially with regard to math background. 25 or 30 years ago, you could probably get into a top 20 finance program with your background, and perhaps even 20 years ago if you had calc 3. Not anymore though.
You might be better off finishing off your music program, then taking the courses in spring and fall 2010 and applying that fall.
What you might want to do is to apply to a few select schools that you would really like to go to this fall and explain to them your situation in your
statement of purpose and that you will take the necessary courses in the spring. Sometimes the adcoms take students under those circumstances especially if the student has research experience and shows good research potential, though it is far more common at the lower ranked schools where they rarely get people who graduated with economics honors from an Oberlin level undergrad, and less common at the top schools where everyone (or at least every serious candidate) either has that type of background or if they are from a lower school, a very extensive math background. If you still have time to take a course this summer (some schools have a late summer session) and think you are able to take calc 3 this summer, maybe you could try that.