You could either sell the K750i and get the W810i (or the K550i) or keep the K750i and port over to T-mobile.

I guess the good thing about having Cingular is that the 850mhz has better building penetration and you get a better signal inside buildings and whatnot, the bad is that there aren't a lot of good unbranded phones that are quadband or triband that utilizes the 850mhz band.