Unless you are playing at ultra high resolutions 1Gb of VRAM is not necessary. You will get the best performance from the factory overclocked 512Mb version
And even at the ultra high resolutions, the 1 gig of ram will still not be fully "effective" since there is only a 256 memory bus. I would personally go for the XT of the ones you listed. It is going to give you the fastest performance.