Hmmm... A HD5970 would beat 460 SLI but the HD5970 has been EOL for awhile now so will be hard to get hold of one, A GTX580 would be almost as good, and in some games where SLI doesn't work as well then a GTX580 would be faster.
There are some advantages to choosing a single card solution, such as:
1) Takes up less room (only uses one PCI-E lane).
2) Often less power consumption and so, less heat
3) No Hassle with SLI not working correctly, waiting for SLI updates for the latest games....
4) You would need an SLI comptible motherboard.
There's one major advantage to SLI though, and thats Price Vs Performance, 2 x GTX460's may set you back £300, but a GTX580 may be £400+, So in this example SLI is cheaper and performance is very similar.
EDIT: I should add that a HD5970 is techically crossfire underclocked HD5870's on a Single PCB, it has some of the advantages of a Single GPU solution, such as only taking up one PCI-E lane, but it uses alot of power, and as its crossfire you may have some driver related issue's
Don't bother with cards that use sli or crossfire on a stick, the reason is due to the pci-e switch and on both brands it is a source of driver and many other issues.
Either go with the GTX 460 option or got get a single high end gpu not a dual gpu card. The dual gpu ati cards are known to have a lot of issues such as one of the gpus failing to a dead switch that leaves one of the gpus useless. The same for nvidia although the switch they use is better but it has a lot of overhead that is masked.
GTX 460 with sli down the road
Or the best choice of ati that isn't a dual gpu card.
Dual chipped as is in dual gpu. Well three chips actually. Two gpu cores and one pci-e switch. The switch has it's driver and is visible to the bios. It splits the physical path from 16 lanes to 8 for each gpu and that is how it uses a single pci-e slot. The switch is often the issue why such cards do not perform as well compared to two physical cards even in 8x8 crossfire/sli. It makes the latency worse and it is already terrible with any add in slot card. I suggest that you avoid any card from either brand that uses that concept unless it is for collection or experiment. The only such cards that didn't have to many issues that were solid were made long ago by 3DFX and that was only for dual gpu cards while every thing up had many issues.
GTX 460 SLI will best a GTX 480 by as much as 20%.
For a single card, single GPU solution to run faster than the 460 SLI you want the GTX 580.
Frankly though, since you already have the 460's there's really no point in buying a 580. If you put your 460's in the 1st and 3rd slots, even running them at 8x by 8x would be perfectly fine. Any potential performance loss would be pretty minimal.