Hi Tan,
To truly leverage the "Gig" phones you are going to want to use
a 10/100/1000 switch as you nicely noted
Be careful when planning as most 48 port switches cannot
power 48 phones with PoE.
This tool "The Cisco Power Calculator" is really great and will take
all the guesswork out of determining these type of requirements;
http://tools.cisco.com/cpc/launch.jsp
Hope this helps!
Rob