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Within 10% performance difference, I can tell you that my 480 (which is equivalent to the 570) found 2560x1080 very easy to run.
Isn't 480 the equivilent of 580? But the 580 is overclocked more or something
Anyway here's a comparison:
https://www.youtube.com/watch?v=SukjHCEWf_U (4gb 580 but I doubt it gives any boost)
https://www.youtube.com/watch?v=9DFyDuykQkA&t=261s (4gb 570)
I'm having the same quandary OP except I'm wondering when will the 570 4gb come down. Because if they can do a 8gb version for £160 surely a 4gb deal for £130 is around the corner. I doubt it ever makes use of more than 4gb although maybe at higher resolutions it will. But I'm only going to play at 1080p
Still I'd imagine you'd get a better performance with a 4gb 580, which can be had for £180 if you look.
Yes, in a way. Performance wise, you'll get 10%ish more from a 580 than a 480, either at stock because it clocks higher from factory, or OC because it can OC higher more reliably than a 480. E.g. my XFX GTR 480 can only do 1350mhz on core & 2125 mhz on memory, which is closer to stock speed on 580s for core. I see 580s can also more easily achieve 2250 on memory, which is great because these cards really desire more bandwidth.
Imo I wouldn't get 4gb even at 1080p. Vram is so cheap and so useful, you'd be shooting yourself in the foot for a measly savings. Certain games are also very prone to eating a lot of vram, e.g. Black Ops, FFXV, Forza H 5, SotTR etc. As for deals, I don't think so, Gibbo is saying they're already selling these at close to break-even or a loss. Thing about vram is, if you need even a little more it makes all the difference in the world to have it. Would absolutely not recommend anyone go for the lower vram cards unless you simply can't spare the extra £ at all.