Depending on what release you have on the Nortel system, you can convert that to SIP too, that might save you money in the long run. Be aware though, SIP can be hacked relatively easily.
The next question is, how do you intend to set up the SIP phones to ensure you still have all the same capabilities you have now if you get rid of the Nortel platform, ie calling extension to extension, and also what are you going to do for voice mail?
You will have to have some sort of system to do this, wether it is in house or outsourced. As Sympology stated, this is an expensive undertaking, so if you think you are going to save money immediately, you really need to do a lot of research!