For scenery, I'd recommend KDFW (aka, Dallas Fort Worth) because it uses 100% Pure FSX Code, relys on GPU rather than CPU, and is DX10 Compatible so it performs better (Also, KDFW is the first scenery for FSDT to use these and since then (2010), it became a standard for FSDT Airports).
KORD doesn't have that as back in 2008, the idea of DX10, and GPU usage over CPU usage wasn't thought of yet but I could be wrong. Seeing the three airlines you thought of, if the new PC is good enough; you could go with KORD.
I don't know about ImagineSim KATL.
I think it would be recommended to use KDFW because of better performance or if you want to fly all three airlines, go with KORD.