It's a more powerful card, it will definitely give higher frame rates. If I were you, I'd be looking at the 4870 1gb at a minimum though. If you wait for a deal (www.fatwallet.com or slickdeals.net) you can get one for ~$180.
Basically, when you upgrade you want the new card to be decidely stronger than your old card...otherwise, what's the point?
In general, I think it's a good idea to get the best single gpu you can get that's at a good price point. The GTX 280/285 are the most powerful, but you can't justify their cost over the 4870 1gb at $180. It's a steal.
Even then you aren't getting a major ugrade...IE you'll be able to play games on high/very high with the 4870 that you are playing on medium/high now. It's a difference...but is it worth $180? That's your call.