It's a little past time to ad another card. You can add a GTS 250 to SLI, but with new cards coming out next month I say you wait a few more weeks (you've waited this long) and overclock your CPU. If you could overclock your CPU Id say get a GTX 560Ti or a Radeon 6950, but at stock clocks I'd say stick to a 6850 or GTX 460. It depends on the game but I think that would be a good match for your system even if they would be hampered by your CPU in some games. There isn't a point in getting anything more without upgrading your platform, though if you're going above 1080p then perhaps a GTX560 or 6870