Raul, The only way that I know how to do this with a radius server is to use Cisco ACS, you can tie in an IP address to the user login details.
With W2k IAS you could do it by assigning a unique username, password (IKE key) and address pool having one address under the vpngroup settings. This would be ok for a small number of users, but gets unmanagable as the number of users grow.
I've not used w2k3 IAS, so no comment there!
Andy