I'm still using 20 years old Edirol Orchestra DXi / VSTi along with Roland SR-JV80-02 Orchestral card in my JV1010. Still give me those killer string sound anytime. The more you know their character, the more you learn how to deal with them and manipulate them to suite your need. Anyway, for me talking about "realistic" is more about how do you sequence & understand how orchestra works in pieces.

If you have a little money to spend, consider Cinematic Strings. I think they're going going for 300.00. A great string orchestra, and super easy to use. Uses the free Kontakt Player. But, as has been pointed out, for Rock, strings are not as important and you certainly can get by with less.

Just remembered something: If you have Dim Pro, you have some strings there. first, Garritan Pocket Orchestra. A curious thing about that, the inst's sound better in that than in original GPO. Second, Dim Pro has its own set of full strings. Each group (violins, cellos, etc). The high violins sound especially good, I use them now and then, and I do classical. you should check them out.

If you know how to write music - and I mean you understand enough theory to write legato strings etc - then you can't go past Notion by Presonus. The sounds aren't the best in the business, but you can replace those sounds with a higher quality string library down the track - although I never have, because I believe if you know enough audio engineering techniques you can squeeze the sound into something more than passable. Even better you get full orchestral library. In fact, I write out all my pieces in Notion and then either import the midi in for drums, or import the audio files into Sonar if it's strings or brass. What I love about doing this way is you can really make the virtual instruments feel like a real person has played them, which is because you can write in dynamics and use the humanize function to push and pull the time. And Notion is only about 150 bucks Oz.   

I use Independence for filler strings and full orchestral. For different articulations I have a few Kontakt instruments along with session strings pro. Also, there's hardware with 5 Kurzweil modules and a Roland JV 1020 with expansion cards.

The Sonivox string companion is a great package for dirt cheap.

^^ This was going to be my suggestion for a budget string ensemble.
All the best.

There's also Kirk Hunter's Pop Rock Strings which has got good reviews. He usually has a good sale (about 1/2 price)once or twice a year. https://www.kirkhunterstudios.com/products/pop-rock-strings/

I use EastWest, Dim Pro, Native Instruments, and Aria strings, depending on articulation needs and section/solo needs, etc.  In my experience, no single collection fits the bill for all situations.
 
In any case, for rock or otherwise, I suggest you voice and play string parts as their own separate instruments, meaning don't just stick them on as a layer to a piano, but rather, record and voice them with their own distinct parts, to sound more realistic.
 
Bob Bone
 

East West, and if you are looking for warmth instead of realism I use Omnisphere 2 and a Korg Karma.