As someone who ended up being an orchestral junky (oh, woe is me): Really too general a question!
How much $$$?
Synth pads/patches or samples?
Simple patches, articulated samples, phrases, or loops?
What type of string/orchestral section are you after (citing a song or piece would be helpful)?
Solo, ensemble, sections?
Do you have Kontakt? UVI Workstation? SampleTank? Engine? Etc. (a sample player)?
What type of music will you be composing?
How will the strings be used in your compositions (featured, solo, background, ambience, etc.)?
Good news and bad news: You can go from free to thousands and thousands of dollars. Knowing exactly the sound you want, e.g. Moody Blues, T.S.O.P, Dvorak, Beethoven, Copeland, cinematic, Mellotron or Motown, can save you a lot of time and money. Also be aware that, in a piece that is not completely classically orchestral, there are myriads of synth pads/patches that sound great in the mix (and on their own).
You're going to have a lot of interesting fun checking out your options.
Paj
8^)