there is definatly somthing wrong there, my new 4870 gives me 90fps constant with max settings and a 22" monitor.
Did you change from the 8800gt to the 4870? if so then there might be driver conflicts, i had many problems with swapping from nvidia to ati, so in the end i just did a clean install.
but befor you do that have you updated to the latest drivers, i think they are 8.12 (thats what i am using)http://ati.amd.com/support/driver.HTML
here is a link.
alos what are the other componants of your pc, something may be holding the gpu back.
hope this info helps