I don't have a display device I could test such a resolution on properly, but ...
You could try and see how it runs on lower resolutions, turning graphics options off or how other 3D accelerated things run and base your choice upon that. It could also be a driver issue. F12 displays the frame rate in Conquest.
I believe the 512 MB memory should be enough (though you can test that with e.g. gdebugger). The 210 does have a damn low fill rate. That's probably what you have to keep your eye on, so avoid the -10 and -20s.
Going with an 240 or 260 should, I suppose, already improve things, though if it were me I would go for the 400 series then (440 and above), they very likely give better performance/cost.
I tested 2560x1440 window mode on Windows 7 with a GTX 260 and it ran with ~70 fps in a match. Even though I have a desktop resolution of 3360x1050 he still seems to render the parts which are off-screen.
Please take these only as suggestions and educated guesses, I can't guarantee anything.
– Rok 足
Toto, I've a feeling we're not in Kansas anymore.