After a lot of tweaking in the Bios, I finally succeeded in making the game run correctly without black screen. This involved upvolting CPU by 10% (I have a Zalman rad, so there is no risk), modifying the RAM CAS latency from 3T to 2.5T, changing AGP from x4 to x8, overclocking my CG a bit (for the fun) and other esoteric stuff like that.

I am not sure which setting did the trick, although I suspect the CPU voltage was for something.
At least, the game showed me that my config wasn't completely optimized. Nevertheless, I can still easily bring my computer to its knees after building 150-200 units, around 30 brawlers, 30 fast bombers, and 20 heavy tanks make them attack a few pieces of rock. Granted, this is a test situation that may arise only once in a while, but it's not completely unrealistic.
Then the Sim time goes up rapidly, eating 65% of the CPU time, as well as the collision calculations. In the end, it seems that lack of memory prevails, as the game starts to write heavily on the hard disk (I didn't check the memory consumption though, I should have). Because of that, the game started to lag behind, and the sim time eventually raised to > 300%, after which I stopped.
BTW, my system is a T'bred 2700+, 2.3GHz, 512 Mb DDR 266 (it sucks, i know), ATI 9600 Pro.