"can you run it" is an OK site
but the best site for gaming test is this:
GameOMeter
click this link :
http://www.yougamers.com/gameometer/10296/
it'll take you directly to the GTA IV test page
press "Can you play it?" and the site will analyse your PC to see if the game can run on your PC
also it'll give you two minimum requirements :
the publisher's and the GameOMeter suggested minimum requirements
PS: I run the game on a Dual Core E2200 2.2 Ghz, so your CPU should do fine
but run the test to check