One gtx 285 = about a hd6850, so pretty good. You should be fine with SLIed GTX 285s for about two more years. No need to upgrade. http://www.anandtech.com/bench/Product/167?vs=158
See? Your better than a GTX 480 with an SLI configuration. The GTX480 is a very strong card too. So, unless you start gaming like a pro, you will not need any more than two GTX285s. Also, because you mostly do casual things, really, any more than two GTX 285s is a waste.