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"Probably not" is the best answer. 2x 6870s in crossfire give GTX480 SLi a run for around the same price as 470 SLi, but I'd suggest waiting until the 6900s are out.

470 are quicker than 6870's is most benchmarks, but 6870's do scale slightly better in crossfire. 470's overclock massively (by ~30-35%), but 6870 OC's are limited (10-15%). 6870 require less power but you may as well use onboard graphics if power-saving is a big issue.
For £350 SLI's 470's are the best bang for buck currently available. You can always sell the two Mafia II vouchers for ~£12 each making the total price less than £330. 470 SLI outperforms a single GTX 580 by 20-30% and the 470 has better OC headroom.
580 is the best single card option, and offers SLI upgradability when prices eventually fall.