^^ More VRAM will be helpful in higher resolutions, although that's a not a high-end card, it is OK to get 1GB variant, if the price diff is negligible.
its not worth dude. the 5670 is not a big memory-cruncher.it does not need full 1gb memory to perform calculations and so there is no difference between the 512mb and 1gb version even at high resolutions.
search in google.
that's why palit released a 3gb version of gtx 580. because the 1.5gb version was bottlenecking the 580's power (read: memory was not enough for calculations) and was crashing at multi-monitor setups.