Depends on the resolution and settings (higher resolutions and Higher settings like AA, AF etc. use more RAM) that you will be using and the games\programs you will be using -- but in most cases the price difference is only $10+/- so I'd get the 1gb. standard version to avoid it being a problem.
EDIT : @ wiinippongamer - the 512mb. versions have been available since shortly after release (at that time there was a greater price difference which made them a viable option but the cost of an extra 512MB of DDR5 memory is now so small that they no longer make much sense so only one or 2 manufacturers still offer them.) For Example : This PowerColor Model