These video cards can no longer be found on sale without serious problems. The 7970/280x video cards have 3 GB of memory, the 290 video card has 4 GB of memory, which will not allow mining some modern coins, and these video cards have very high power consumption, which is also a problem.
Still better than a heater, power consumption is heat generation, the more power consumed the more heat generated, these cards can still reduce cost of heating if you are in a cold climate and the newer cards are still overpriced hehe. Even better if you already own them.