How can i convert my network to domain ?

i have 150 sys in my company these are conntected with workgroup.i would like to convert all these sys to domain. i have cisco 2500 router and two 40006 switches all these sys are connected via dhcp for internet. I have dell servers and one zenith server with xeon 2.4, 1gb, 120 gb hdd. I have great environment, how can i convert this to domain. and which is the best os or soft for band width management to controll internet connectivity.. Pls help me.. Thankyou..