What to wear up top?

Not sure what replies you got in the other thread but why not a tri top?  If you shop around you can find pretty cheap options.  I prefer the Orca 226 which is slightly longer in the back. 

A sleeveless tri top is the best solution.  They're designed to be worn for the entire race.  As disussed in the your other thread, the last thing you want to do is to try to pull a dry shirt on over your wet torso in T1.  Almost impossible and a big time waster, especially in sprint races.

Nothing is going to be perfectly dry by the time you get on the bike so you may be a bit chilly at first, but they do dry out remarkably quickly. 

A tri top should fit snugly; you don't want it to flop around and create excess drag either during the swim or on the bike.  Most have front zippers that you can undo to help cool down on the run.

In the past I've used Zoot and 2XU tops and currently have a Louis Garneau top that I like.

Mark

If you do go the tri top route, pay attention to the one you get.  Before I started using the tri suit I picked a random shirt/short combo in my size at the local shop and the top's pockets were really loose causing all kinds of drag during my swim.  I never trained with the top on, so when I started my swim during my second race I was in for a huge surprise (put the top on during T1 my first race so didn't experience the drag).  After that I switched to a tri suit.