I've noticed that alot of games wont run at all with OC'ing. I have a few games that even when using Catalyst Overdrive, they will either crash upon start, or if they do start, will have horrible artificing. You should have no problem playing AoEIII with your 1900 @ stock speeds. Even with the settings maxed out I would imagine.
Games that I've found WILL NOT run when OC'ing...
- Dungeon Siege 2
- Gal Civ 2
- Sid Meier's Pirates!
- Joint Task Force
- Ghost Recon: Advanced Warfighter <-- can be kinda picky about the exact clock speeds.
Of course, this could just be symptomatic of MY system. These games may or may not run while OC'ing on your system. I'm in no way stating that these games will never run on any rig while your card is overclocked. For example, I do remember that when I had my X1600pro installed, I never had any problems running Pirates! or J*T*F while the card was overclocked. But now that I'm running a X1950XTX, they wont even load! My cousin told me that his X1800 has problems when playing OpenGL games while OC'd. This is definetly a problem that catches my interest... thanks for bringing it up