Just about everyone knows that Nvidia’s hot new Fermi graphics chip is literally hot. So, when Asus bundled its new ENGTX480 card with a custom voltage tweaker for overclocking, we wondered if it was such a good idea.

After all, do you really need the card to run hotter? And with the speed of the ENGTX480, you probably don’t need the higher clocks anyway. The ENGTX480 ships with 32 shader processors (what Nvidia calls “CUDA cores”) disabled, yet the card still manages to be the fastest single-GPU card you can buy today.

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We’ve never been major advocates of GPU overclocking, as the minor gains you achieve often don’t justify the added heat and instability. But there’s a clear difference between Billy Joe doing a maximum overclock on his GPU and a vendor overclocking the part at the factory.

So when XFX offered up its XXX Edition of the already-fast Radeon HD 5870, we were naturally curious. XFX pushes the HD 5870 to 875MHz (3 percent over the stock 850MHz) and juices the memory to 1,300MHz (8.3 percent over the stock 1,200MHz). At first blush, a 3 percent core overclock seems minimal. Given that the card costs about $430, versus about $405 for the stock XFX variant, is it worth the extra jingle?

To find out, we compared the performance of the XXX Edition to a standard XFX Radeon HD 5870, which is a stock card in every respect. Save for clock speeds, the two cards are identical: memory (1GB), ports (two DVI, one DisplayPort, one HDMI), and the reference cooling system. Because of the speed bumps to the XXX Edition’s core and memory clocks, its system idle power usage varies from the stock card, reaching 148W versus 141W.

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The recipe: Take two of the fastest GPUs on the planet capable of running DirectX 11, specially chosen for their low voltage leakage. Toss in two gigabytes of high-speed GDDR5 memory. Mix all ingredients into a card with high-end Japanese solid capacitors and a souped-up thermal dissipation system. The result: the XFX Radeon HD 5970—a GPU so yummy, you may even go back for seconds.

While the product name doesn’t hint at the card’s dual-GPU nature, there’s no mistaking the presence of two graphics chips when you check out the back of the board. Then there’s the sheer size of it: At more than 12 inches, you’ll need a high-end PC case that’s deep enough to handle this monster. You’ll need a beefy power supply, too, since the HD 5970 burns 294W at full throttle—and that’s if you don’t overclock it. The good news is the card consumes just 42W at idle, less than double the idle power of a single HD 5870, thanks to an enhanced deep-sleep mode for the slave GPU.

Continue reading this review after the jump.