If you want to estimate anything, check the graphic performance websites like www.guru3d.com
and see how they compare cards performance. If for example the 8500 is on the average slower than our recommended cards, you can assume that the performance you'll get from SSM will be also slower than what we publish. That of course also depends on the motherboard, RAM,CPU etc. so as you see, it is impossible for anybody to give you any educated speculation. Sorry.
Also read our Hardware FAQ for more info.
We also recommend not to buy any upgrade at this stage. Run the game and see if it performs as you want. If not, then consider an upgrade.