GDDR3 vs. GDDR5: Is it a big difference?

I recently bought a 4870 to replace my 4850. What are the main benefits of faster data rate RAM? Does it help at higher resolutions? Does it help with AA? How so?

http://www.playtool.com/pages/vramwidth/width.html

its a little bit outdated but the ideas inside are the same

The great thing about gddr5 is that its quad data rate allows it to effectively emulate twice its native bus width compaired to gddr1-4. This allows you to use a smaller bus on dye while not bottlenecking the card. The benifits of this is multi fold. Basically using a smaller bus on dye saves space, produces less heat, makes it simpler to design both the gpu and pcb, and best of all makes it cheaper to produce.

The basic reason for high bandwidth is not to limit the gpu. Usually cards with low bandwidth drop off more at higher res and with more AA compaired to cards with more. However you reach a point of deminishing returns that depends on the gpu architecture itself and thus can varry widely through generations and companies