Very odd crash.

I yesterday got an ATI 5750 for my desktop and tried it on Napoleon: Total War. I did a random battle, and it was running smoothly at 70 FPS. I start the battle, play for two minutes, then the monitor turns off and says "Monitor going to sleep" and through my headphones I hear a hum. I was unable to shut down my computer so I turned off the power. I tried the game again today and played an MP battle and it didn't crash and the battle finished perfectly fine.

Should I be worried about the first crash earlier?

have you used the troubleshoot thread? in there you will see how to properly install gpu drivers, this might be the issue here.

Exactly the same happens to me - dealing with it on the random black screen thread - I thought it may be either a PSU or fan problem, but I will add gpu drivers to the list of possibilities...

This is a very strange issue. I just played a few hours of N:TW on MP, and nothing happened, but the moment I switch to SP...instant crash.

Is it a game-related issue?

As run out of memory? Possible. Can you start an SP campaign now after the crash?

Is it a ram problem? I've got 3 GB of it. Figured it was enough.

depends on the OS, try to take a look at the madboris thread

It's not using all 3 gigs with 32bit. MadBoris' thread will get you the max 32bit will allow. You want utilize all 4gigs, 64bit install.