Now it's been a while, and I don't have a book in front of me, but I believe this should work:
**broken link removed**
Gain on this circuit is: Av = 1 + R2/R1
So if you have 2.5V and want 5V, Av = 2. Thus R2 and R1 ought to be equal.
Hope I got this right, and that it helps.