abacab
The free included Cakewalk Studio Instruments Strings is all that you probably need for pop/rock backing tracks.  Next step up for cheap might be the Sonivox Orchestral Strings, available for $29.99 until April 2nd.

I was just about to suggest both of those. Studio Instruments Strings are surprisingly good. Sometimes you can catch the Sonivox strings on sale $19.
 

The SI Strings is amazingly versatile - especially for a bundled instrument. It covers the pad spectrum at low-medium velocities, and the bows really seem to "dig" at the higher velocities.That can actually be a problem sometimes, for example if you just want a basic fill that doesn't stand out, but your keyboard controller is touchy. SI can feel like it's wobbling between sticking out and dropping out. For some tracks I've used MIDI FX as a sort of "MIDI Limiter" to narrow the key velocity to just the range that the part really needs - works like a champ.
 
But as others have said, SI really shines when it's layered with other parts.

 No joke, abacab.
 
Though, I think I caught a whiff of "stinkin' rock track" in that remark, but I nonetheless acquiesce and will strike that from the record, your honor.
 
Like a handyman friend of mine once said, "You want the right tool, for the right job, at the right time."

Except for that first Genesis album (From Genesis To Revelation) where the strings were WAY overdone...