You should go for the least common size IMHO. Devleping for
both the company I work for (Govt./DoD contractor) and personal
projects I go with 800x600, though if I can I make sizing dynamic.
In fact, some contracts require devloping at 800x600. I wouldn't be
too suprised if there is somebody somewhere still targeting 640x480
;-P. You should first determine your target audience and do whats
best for them of course.
For the last couple years we’ve been using 1024 x 768
for the vast majority of our corporate clients (business to
business projects) as well as for projects intended to be released
to the general public. We’ve got one large educational (K12)
project that is still 800 x 600 … partly because of the cost
that would be involved in upsizing the legacy 800 x 600 design,
also party because schools are sometimes far behind even the
general public in upgrading equipment.
Just a quick word of advice: be careful with your test for
screen resolution … I’ve seen lots of projects that
only test the primary monitor … I’ve got three two
monitor systems here in the office and, while not the norm, two
monitor systems are far from exotic these days.