The RV740 is ATI's first 40nm chip. From what we know, it should be already sampled, while the official launch should be later in 2009, probably Q2 2009.
The chip does have 256-bit memory support, and it does support GDDR5 memory, as well as GDDR3, and it is designed to sit between RV730 and RV770.
It should be close to RV770LE performance, so it will eventually replace Radeon HD4830 parts, and put more pressure of Geforce 9600GT. As far as we know, ATI should be the first with 40nm, while Nvidia will follow.