Chosing R and C values for a given RC filter design
Sallen-Key filters are very gain-sensitive!
That means: A small deviation from the required (fixed and finite) closed-loop gain Acl of the active stage will cause relatively large deviations of the desired Q-factor of the filter. 

Therefore, two Sallen-Key alternatives are preferrable: Acl=1 (feedback path is a short) or Acl=2 (two identical resistors in the feedback path). Do not use the "equal-component" form of the Sallen-Key structure (R1=R2, C1=C2) because the required gain value is uneven (not easy to realize).  

For designing the passive RC-network follow the recommendations mentioned in the other answers.

Recommandation (edit):

1) Unity-gain version: In case of Butterworth response (Q=0.7071), two equal resistors are possible (capacitor ratio of 2)

2.) For gain-of-two (Acl=2): Two equal capacitors are possible (for all Q values) 

3.) When a second opamp is available, you can decouple the first node from the rest. In this case, you can use two unity-gain amplifiers and have the same properties/advatges as in case 2) (equal capacitors).