I'm doing a ton of research on this actually. From what I've gathered, the phone should work where you have GSM 1900 service. It will not work on GSM 850 networks obviously. And yes, your current SIM card should work fine in the unlocked phone.
If you go ahead with the purchase, you should post your results.
I have the k750i on Cingular in NJ and it works for the most parts. Sometimes in the back woods I have one or two bars but never had a problem making a call. Using 1900 only though gives me problems sometimes in steel buildings or underground like basements. Im willing to make those sacrifices for a phone full of features that actualy work. Use to have a v710 from Verizon. Great service but crappy phone. Now I got a great phone and ok service.