should be good. if anything, it would probably be the other way around, especially with cpu intensive games like gta iv.
gta iv gives you a bunch of options and lets you know which ones depend on which component, allowing you to turn down the ones that really depend on the cpu and lets you turn up the ones that depend on the gpu. depending on your monitor, i'd say graphic options would be able to work fine with the 5770 on medium and possibly even high on some. but you'd probably have to have low settings for the ones that are cpu intensive.
the 5770 is an excellent card for the money.
1) what is your current setup?
2) assuming you haven't bought it yet, what kind of budget do you have?
3) what games are you going to play?
you should be able to see some performance increase or at least be able to have higher graphics settings (the ones that depend on the gpu) while having the same or better performance.
for the rain, this is just speculation, but i would assume that is due to the cpu having to render more things per frame.
Okay, I bet It'll deliver just fine, thank you for your help.