help to divide bandwidth

i need to divide bandwisth in 2..
i have 2 mbps and i need to separate 1.5mbs for surfing and 0.5mbps for voip…
i have nat-masquerade my public ip in my interface.. i was thinking of adding another lan card and do masquerade also and have the following,

public(ether1)
^`
private(ehter2) voip(ether3)
(1.5mbps) (0.5mbps)

so if anyone could help on how to do it or either another efective way to separate my bandwidth

I would make use of queues and simple queues.

Please consult the Mikrotik manual for info on how to use simple queues. You could use Mangle rules to mark your SIP (VoIP) traffic by means of port, and give priority to those connection marks. It is also possible to let your internet traffic utilize the entire 2mbps line, until a VoIP call is made. That VoIP call will then have priority over the first 512kbps segment of the line.