AoE2 System Requirements:
- G3/350 or faster (500MHz required for on-line play)
- Mac OS 8.6 or higher (Cabonized for OS X)
- Rage 128 or better video card
- 128MB RAM
- 750MB free hard disk space.
Civ4 requirements
- Mac OS X 10.3.9
- PowerPC G5 or Intel chipset, 1.8 GHz
- 512 MB RAM
- 3.5 GB free disk space
- Supported Video cards:
NVIDIA GeForce 5200, 6600, 6800, 7800
ATI Radeon 9600, 9650, 9700, 9800, X600, X800, X850, X1600
MacBook Air specs:
Intel Core 2 Duo processor, 1.6 Ghz
- 2GB of 667MHz DDR2 SDRAM
- 80GB 4200-rpm Parallel ATA hard disk drive or Optional 64GB solid-state drive
- Intel GMA X3100 graphics processor with 144MB of DDR2 SDRAM shared with main memory
There's a possibility you could have an issue using integrated graphics instead of a dedicated/supported video card, but integrated graphics have come a long way since these games were developed.
You also need to keep in mind that these games (AoE at least) require you to have the DVD in the drive to play, for which the MBA only has an external option. Sure, you could install a "no cd hack", but...
You may want to consider whether or not the MBA is for you to begin with. It's really not meant to be your primary computer or portable desktop. It's a lightweight, easy access temporary computer for when you have to step/drive/fly away from your main machine.
I have AoE2, AoE3 and AoM loaded on my MBP and they run great. I don't mind the extra weight because it is my only computer and a capable portable desktop. If I already had an iMac, I'd definitely consider a MBA. But there wouldn't be any games/music/movies/pictures on it; that's what the 16GB USB stick is for...