I am in a similar situation with a 1gb radeon 5850. I read something last night that said there may be 4gb versions coming out at the start of March. I don't know how much salt we need to take with that though. I can't find any info about the ti version at all. Not sure I can wait much longer to be honest!
4GB would be absolutely pointless on this card with 128 bit interface, as it has nowhere near the GPU power to play games where 4GB comes into use such as heavy textures or high resolutions. So you'd just be wasting your money