LestaT: No way. Your RAM and CPU are scraping the edge of acceptability, and your graphics card will barely manage lowest settings.

I'd recommend upgrading, and in this order: New graphics card (at least a GF 6600), 512MB more RAM (1GB if you can afford it) and a new processor (go for an Athlon 64 of some kind).

Doing all of the above should come to about 300-450 Euros, and will allow you to run the game as you described (medium settings, max unit size).

P.S.: Are you a Muslim? Just wondering cos your sig is a Qur'anic excerpt if I remember correctly...