Have you double checked that SLI is enabled from the NVIDIA Control Panel ?. As the GTX 295 is 2xGTX260’s I believe you should have this checked. If you update the NVIDIA drivers it usually turns SLI off ( well it does for me anyway). As a note: SLI does work for the game as both my GTX 460’s use the same resource whilst playing. SLI is just not perfectly optimised yet.
All 4 cores are used on My PC ( old Q9550) and the game is CPU intensive. How did you work out that it was only using 10-20% cpu. If you ALT-TAB out the game to Task Manager then you will not see the true CPU usage at that point in time. You will need to look at the four CPU’s history graph under the Performance tab to get an idea.
Hope that helps.