Two issues with what you're asking for. 1) You want something aero for TT, but anything too aero won't be great for group rides if there are any crosswinds. 2) Your budget will limit you for anything very aero. Deep carbon rims will run you considerably more than $600.
You're really looking for 2 different wheelsets. But if you need to compromise, see if you can pick up something like the reynolds assaults used on ebay or craigalist. Alternatively, you might want to save up and pick up a new pair of Williams 58s. They're pretty sweet and very reasonably priced.
FWIW, I use my road bike for group rides and tris and I have a set of ksyriums and williams system 30s for training and group rides and switch to a set of 808s when i race tri.
Maybe something with 27 or 30mm rims would be good. I would say 24f/28r spokes, but if you want to save them for special rides and use your other wheels most of the time 20/24 will be okay. Here are some sample pictures with 27mm rims and White Industries hubs:
You'll get more aero benefit per dollar with (1) a good TT fitting on aero bars and (2) an aero helmet. A skinsuit would also help more than wheels on a per dollar basis but may not be practical for tris.
When you replace your tires next spring, get some thinner racing tires (I like Michelin PR3s). They won't last quite as long as Gatorskins but they will roll a bit better, saving a few watts of rolling resistance.
Keep your current wheels for training and group rides. With your $600, get a sram S80 front wheel and Wheelbuilder disc cover for your rear training wheel.
The disk cover is a good option. But if it is windy on race day it will probably be back to the original, unaltered wheelset.
I don't see SRAM wheels very often, but from the people I spoke to they are happy with them. I just don't like the radial lacing on the rear drive side of any wheel. For this option it doesn't matter since only a front wheel will be used.
As was previously mentioned, getting a good fit on the bike will probably be more beneficial than the wheels. If you are concerned that the wheels are poorly built you can drop them off at a good shop to take a look at them and verify the tension is equal. (Or you can pluck the spokes and expect the same sound to get a good idea of the build quality) If they weren't stress relieved properly you would have found out on your first ride.
If they weren't stress relieved properly you would have found out on your first ride.
So 70+% of the bikes sold at LBS's, typically having machine built, cheaply spec'ed wheel parts etc. that have not been stress relieved during the build have been failing on customers on their very first ride? Sort of an amazing statistic really.