Software Rendering and OpenGL Textures

I have been told that the bus between main and video memory is too slow to allow for constant updates to large textures. This seems like it would rule out, say, rendering a screen of a game in software then dumping it to the video card via OpenGL for postprocessing.

However, it is my reasoning that an emulator developer should want to do exactly this. An array of pixels would be easiest for an emulator to work on. Most emulators offer a DirectX or OpenGL mode for better sync, which makes me wonder what's going on underneath.

I'll get to it, then: Would it be feasible for a 320x240 game to be rendered in software, with the screen updates pushed to video  memory on VBLANK for postprocessing (upscale, scanlines, etc)?

I think it shouldn't be any slower than framebuffer readback, which even though it's slow is used by ENTERPRISE renderers such as Rage.