i have mixed gpus in one of my rigs, a 290x, 270 & 2 8750's, only a couple cards would work with 4gb system ram, after adding another 8 they all started, i could be wrong, but it couldn't hurt to add another 4gb if you have an extra slot & a stick of ram... or even spending $40 on a stick, better than your $$$ sitting dead.
it's not sitting dead, i have it in my gaming pc. but it's not ideal, since my gaming pc is nvidia based. even getting both drivers installed at once was kinda hectic.