Ive played around with this a bit and RTW seems vastly more processor based than anything else.

Playing multiplayer games with only Humans (no AI) I can get smooth battles with up to 8,000 units. 8,000-14,000 runs OK. It's playable.

Add in an AI or two and my computer starts to chug at 8000 units.

In a custom 3v3 with all AI controlled players I get around 10 FPS, pause the game however and everything runs smooth. Changing video card settings does absolutely nothing and the game is only using around 300MB of RAM.

As for what 15,000 looks like. It looks ok but not that "epic". Mainly because its 6 different factions with all different colours, all moving in different directions. It looks like a swirl of chaotic colours with no direction.

The dead after the battles looks like someones thrown up all over the map though, that looks cool.