Which is better, for Revit specifically? I can get nVIDIA GTX 500 or 600 series cards in either 1GB 256-bit memory, or 2GB 128-bit memory, each for under $200. But going to 2GB 256-bit memory pushes the price up considerably, well above $300. Any thoughts?

*edit* this would be for my home machine running Revit for me to do work after hours or on weekends. It's not for full-blown day-to-day production work, but I could see myself running a rendering on it from time to time.