RAM Speed + Bitwidth = Bandwidth (a little more complex than that, but that's the general idea)
Basically, the 1792MB Version of that card was designed to look impressive, but like the 2GB HD4890, it falls on it face as it does not properly use that much RAM for even the highest resolutions (2650x1600)
Having >1GB of vram is really only useful if you are going to run SLI or tri-SLI and have at least a 2560x1600 resolution. And even then, it's only sometimes going to matter (on certain games that use exceptionally high-resolution textures like ... you know ... Crysis)
Otherwise you're better off with the standard memory amounts on either a 275 or 285. The extra memory bandwidth does help a little on the 285, as does the 32 ROP's (vs 28 on the 275) ... but all in all the 275 is pretty dang close performance-wise to the 285, and thus it's a much better deal.
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