Unlock phones would give you the same results as locked phones. Naturally, there are some that would give you 'more/less' signal coverage. But that has to do more with the manufacture of the phone than the actual signal coverage. If that makes any sense. I currently have an unlocked V360 for my Cingular service. Only difference that I can find is that I have to make all the set ups myself for texting, web connections, and so forth. Those directly from the Cell Service Provider are done for you. That is about the only difference. So just to answer your question, my unlocked phone gives about the same, if not better, signal coverage compared to any other brand name phone that Cingular can push out.
If you are sure that your cell phone that you want matches with your old one (850/18008/1900), then you should be fine. Most GSM phones in the US vary between the 850 and 900 MHz, where as the rest of the world uses the 1800/1900 MHz. All you have to do is install your original SIM card to the new D600 phone, and will work.

I hope that this helps and good luck.