Well the r7 240 ddr5 is quite good.
But if you can buy a 5850 most probably you can afford a r9 270 and a fx 6300 and a MB 970 chipset. All these end you up at 350$ soo its mostly enough.
The r7 240 doesn't even support crossfire.
The HD 5850 does support crossfire but i don't think it would crossfire with your APU.
And why use old cards? The r7 240 is not old but it's not really good for gaming
Well the r7 240 ddr5 is quite good.
But if you can buy a 5850 most probably you can afford a r9 270 and a fx 6300 and a MB 970 chipset. All these end you up at 350$ soo its mostly enough.