Catalyst Control Center

You may want to do scaling on the GPU and set it to aspect ratio. You can do this on Nvidia, I assume AMD has similar settings. Usually this won't be necessary as long as you run at native resolution (1280x800) but sometimes it can help.

So how does native resolution with a high level of antialiasing compare with a high resolution and no antialiasing?

Depends on AA technique. To note here, downscaling on GPU/Rift Box is equivalent of supersampling. Multisampling for the Rift usually gives poor results, as it discards large amounts of subpixel data. Supersampling doesn't discard any data and perceived best in the Rift, although it's more expensive than MSAA. Don't even bother with fast approximate anti-aliasing (FXAA) - it takes little per-pixel information and makes it even lower (by extrapolaton ditherng) for the sake of slight smoothing of hard edges.

I've figured than settings in CCC have no effect in Rift mode, at least in iRacing, so I just keep global settings, which are "everything fastest".