Sitting in a tower will drain different FPS based on what map, which custom objects, how many trees at the end of the field tile in the direction you're looking, and the presence/absence of airplanes that are being rendered but aren't visible (like planes just behind your tower's roof, but if you adjust view they're still there. They are still impacting the system a bit.
AH is a CPU intensive game. I've upgraded a few times while playing it. By far the best change I ever made was getting off the old clunker P4 and moving to a Conroe C2D. See if your motherboard can support the newer CPUs. Even the lowest-end dual core is going to outrun your older P4 3.06 with EASE, and that translates into better FPS. As for other possible issues: Your video card is more than adequate, and wouldn't be the problem. Your RAM is better than adequate. I ran on 512MB for a while, then 1.5GB for a while, then down to 1GB and now have 2 GB. Moving frmo 512 to 1GB or more will DEFINITELY help AH, but you already have that much. I don't notice much improvement going from 1 GB to 2 GB (inside AH, that is. I notice some improvements on other programs in my system).
Your main bottleneck is your CPU and/or possibly your motherboard at this point in time. My $0.02