I got a dirt cheapo Plantronics headset from ASDA. Used it with two Nokia phones and a Motorola. Works a treat. Only problem is the charger has a different connector to the phones (although Nokia's would charge from headset charger, other way round wouldn't work)
My best advice from experience of this and a Nokia headset (Mrs Ghost has one, and won't let me use it!) is to get the same make as your phone and make sure the charger is the same and if you have a car charger for the phone, that it charges the headset also.
OEM Phone branded ones are much more expensive though!
I did have a Nokia CARK91 car kit in the car, you could buy a bluetooth adapter cradle for it to work with other phones, but when you add up the cost of this and the leads you need to get it to work in an Omega, its much better to get a headset that you can transfer between cars. Or buy a TomTom with BT handsfree.