i dunno...depends what you expect from something...of course like...for example if you had an 8800Ultra with only 128mb of RAM...it would still be reasonably fast...so for the faster processor graphics cards, 512 is more than enough....
well, it always improves graphics + speed(fps) when displayed on higher resolution screense...however, i feel a 1gb Graphics card is not necessary....but projectors' true displays are only 1280X1024 or something...so it doesn't matter even if it's displaying a few hundred inches
but i'm on my laptop and i'll proove it when i get my Thermalright Ultra Extreme next week...when i don't have to run it over 75 Celcius...its dangerous, i put it on 3.2Ghz...but it went till 80C after it finished the Pi calculations....i dont wanna do it again