Buying a non reference card wont give you more hashrate then the reference models. Maybe some if the card is overclocked from the factory but hardly worth the extra cost.
Reference 480 8Gb can give you around 27Mhs (underclocked) without a bios mod.
Reference 470 4Gb give you around 23Mhs without bios mod.
With custom bios
Reference 480 8Gb 30-32Mhs
Reference 470 4Gb 27-28Mhs
If i where you i really should consider going 470 + custom bios.
Reference 480 8Gb can give you around 27Mhs (underclocked) without a bios mod.
Reference 470 4Gb give you around 23Mhs without bios mod.
With custom bios
Reference 480 8Gb 30-32Mhs
Reference 470 4Gb 27-28Mhs
If i where you i really should consider going 470 + custom bios.
The price difference between the 470 and 480 is about 20%. So it might be good to buy the 470 to have higher density.