Value and Conclusion

Since the benchmark scores are all over the place I'll walk you through them.
We tested 1x ARES, 2x ARES and for additional reference ARES+HD 5870 and ARES+HD 5970, an additional data point is provided by 2x HD 5970.

• ARES + HD 5970: This seems to be the worst combination. The Radeon HD 5970 has only 1 GB of memory per GPU while the ARES has 2 GB. So in the CrossFire setup each GPU can only use 1 GB which means that there is no benefit from the extra memory on the ARES. A similar situation happens with the clock speeds. Being clocked at only 725 / 1000, the HD 5970 holds back the ARES that is clocked at 850 / 1200. As a result the performance of using ARES with a HD 5970 is almost identical to that of two HD 5970 cards - avoid this combination!
• ARES + HD 5870: This setup works considerably better. Both cards are clocked at the same frequencies of 850 / 1200 which wastes no clock cycles. There is still the difference in memory size (HD 5870 has only 1 GB), but with only a few benchmarks out there that profit from the extra memory, the losses are rather small. As result the 3-GPU combination of ARES + HD 5870 is actually faster on average than the 4-GPUs of ARES + HD 5970. If you really want to run ARES in a CrossFire setup but can't afford two ARES cards this is the sweet spot to go for.
• 2x ARES: This is clearly the optimum solution, but also the most expensive. Compared to the previous configurations we only see noteworthy gains in Crysis, Stalker: Clear Sky, Metro 2033 and Unigine: Heaven. It comes as no surprise that these are also the only titles in our benchmark suite that run at below 100 FPS when played on a dual ARES at 2560x1600.

Ok, you already know from our single card ARES review that running one card isn't reasonable for the average user. So what about two of them? Coming back to the car analogies: if you already have a Ferrari why would you buy a Bugatti Veyron? Probably simply because you can.
It's a bit similar with the ARES. Outside of extreme world-record seeking overclocking & benchmarking I can't imagine anyone who needs that much processing power. It should also be noted that many games won't gain that much with only a few exceptions.
Mismatched combinations of the ARES with other HD 5000 Series cards do not seem to make much sense with the only exception being the HD 5870. Both ARES and HD 5870 share the same clock speeds which helps with CrossFire performance yields. Another interesting property of this combination is that it will work great in most motherboards. Running two ARES cards requires a spacing of two slots between the cards, with HD5870+ARES you can get away with a single slot spacing motherboard. Put the HD 5870 in the top slot and the ARES in the next available PCI-E slot. At "only" $1400 this is also cheaper and offers almost similar performance.

Running ARES CrossFire is not done by getting two of those cards. As mentioned before you will need a motherboard with enough spacing between the slots. Next you should have a fast monster CPU, our 3.8 GHz Core i7 is CPU limited in some benchmarks with dual ARES. So better look for something in the 4.5 GHz range. We already talked about monitors in the single card review, without a 2560x1600 30" display or an EyeFinity setup there is no need for the ARES. Finally, make sure you get a good power supply with loads of connectors. Our testing shows a maximum power draw per card of 450 W which means you should plan with 800 W for your CrossFire setup, add CPU and the rest of the system on top of that and 1200 W is a reasonable minimum to go for. The requirement of 4x PCI-E 8-pin and 2x PCI-E 6 pin power connectors is something not even all 1 kW+ power supplies provide, make sure to check on that while shopping.