First for understanding how much GPU memory you need, you need to understand when that memory it is used.
The GPU memory is used to store the frames generated. Now, the size (MB) of those frames varies depending on the actual resolution and the effects used on the frame. Bigger resolution means higher size for the frame. Same goes for details like AA, AF and so on.
For resolutions up to 1680x1050 a 512 MB card will be enough even with the highest details enabled. Higher resolutions will need more RAM.
Seeing that you use only 1280x1024 resolution I would recommend the 512 version, but if you plan to change the monitor in the future for a bigger one or the 1 GB version is only a little bit more expensive then go for that one.
First for understanding how much GPU memory you need, you need to understand when that memory it is used.
The GPU memory is used to store the frames generated. Now, the size (MB) of those frames varies depending on the actual resolution and the effects used on the frame. Bigger resolution means higher size for the frame. Same goes for details like AA, AF and so on.
For resolutions up to 1680x1050 a 512 MB card will be enough even with the highest details enabled. Higher resolutions will need more RAM.
Seeing that you use only 1280x1024 resolution I would recommend the 512 version, but if you plan to change the monitor in the future for a bigger one or the 1 GB version is only a little bit more expensive then go for that one.