There's a definite tradeoff here. Buying an nVidia and using their binary
drivers would (probably) make things work well now, while an ATI with free
drivers might have problems now. But with the ATI card the driver is
likely to only get better with time, while with the nVidia card the driver
is likely to get harder to make work.
So it should be interesting to see how things are with the same card
running next year's software. Or even this fall's software.