It should be able to run both games at low settings. I would not buy both games at the same time. Go with FO3 first since I've seen numerous people complaining about FO:NW performance on systems more powerful than what you have.
The minimum requirements for FO3 is a Pentium 4 2.4Hz or equivalent and a GeForce 6800 / X850. The Radeon HD 4650 is slightly more powerful than the X850. However, those are desktop specs, not laptop specs.
Generally speaking, laptop video cards are about 10% - 15% slower than the desktop version.