Good walk around lens?

Hi there,

I have been using 50d for a year now, it is my 1st DSLR & I'm still in early stages of learning? When I had bought it I got it with EF-S 60 & 200/2.8. Its been a while since I realised that for portraits 60 mm is good but when I'm trying to shoot pictures on some function/party its completely useless.

I ve been considering some zoom lens for such occasions, I 'm looking for something below 500-600$. I know canon has some really good ones- 24-70, 24-105 but both are expensive.

Is Sigma 24-70 any good & anyone has any idea about the new version. Any other advises on lenses which could fit my needs.

Thank you