I am thinking to buy a VoIP telephone to replace my current landline
phone. I wonder what are the things I should consider before buying
one? I googled a bit and found that all the phones have basically the
same functionality except for the extreme high end ones.

Basically your internet speed. If you have an old skewl 56k modem and
your trying to run VOIP then youd be pretty screwed. However, if you
have a 2mb connection then you will get fast speeds and be able to get
better quality.

thanks for the help. ^_^