The replacements still available are the Apple firmware 5770 and 5870. But the 5770 requires on aux power cord and the 5870 requires both aux power cords. Either of those cards can run three displays, but you must follow all the rules and use just the right adapters to run three.
The GT120 was the standard and add-on for the 2009 model. It should not really be having problems, unless your Mac is dusty and is getting too hot inside, or one of the fans is not spinning. Each GT120 should be able to support two displays by itself.