Having recently bought a Dell 3000 Dimension for £300 with 2.8G processor, DVD reader, CD writer, 512Mb RAM, 80G HD & 17" TFT, I would say, go for something like that.
I have always built up my own systems for the past 20+ years, but this runs all the usual stuff nicely, including FS2002, Age of Empires, Age of Mythology (since you mentioned that type of game).
The on-board graphics, which I have never considered before, are up to the job.
It's not a machine for much expansion but if the spec suits your requirements it's pretty good value imo.