My machine:

Intel Core2Extreme
2GB RAM
8800GTX
X-Fi sound card
plus other bits

I run the game 1680x1050, with all graphics setting maxed out (including extreme shadows) and I use huge units.

I have the "unlimited_men_on_battlefield = 1" set and never see the reinforcements delated message. (though things can get choppy after the 4th full army...)

If I switched "unlimited_men_on_battlefield = 0" I would see the message after the 3rd full army...