You are safe developing for 800x600 and above. I still have a few users that use it (for eyesight purposes - I am getting there myself

), so I code for usability at 800x600 and optimal viewing at 1024x768. This covers about 99.9% of users - most using the larger size.
Forget about 640x480. If you lose a customer because of it, that would be good news actually, because it means you have already sold a million copies of your software before you found that guy!
BTW, your audience with this poll probably has much better equipment than the average user, so unless your target market is programmers/professional programmers (I'll start another poll to answer this), I don't know how useful it will be.
Cheers.