60D Body - Lens recommendations for a novice: $500-$600 Budget

Hello, I'm a novice here - just purchased my first DSLR, a 60D in kit form with the EF-S 18-135. After reading some reviews it would seem that there might be some zoom lenses out there with better IQ in a similar price range. I would like to sell the kit lens (it looks like I can get around $300 new?) + plus I have another $300 to spare, giving me a total of $600 to spend on glass.

I would like some zoom capability, no less than 55mm, ideally closer to what the 18-135 offerred. I also wouldn't mind a faster prime lense for portraits, low light, video, etc. I will be using the camera to do some video as well.

Would it be best to spend the entire amount on 1 zoom lens? Or can I get a decent zoom lens for $300 plus a decent prime for $300? Someone recommended a Sigma 18-125 - thoughts on that one?

Depending on your thoughts above, what lenses would you recommend in that price range? I really wish I could spend more, but that's my budget for now.

I'm sure I would be initially satisifed with the 18-135, but I can see myself growing out of it quickly and wanting something better, so I'd rather purchase it up front.

Thanks!!