Depends on the full configuration of your computer and the settings on which you want to play GTA IV.
The results in the two sites differ because both benchmarks use different settings, the one at Tom's Hardware uses:
1280x1024 (Medium settings, Vsync off)
while the one on behardware:
Once again all the settings were
pushed to a maximum, with the
exception of the textures so as not to
exceed available video memory, all at
a res of 800x600.
I think any of the two family of processors can handle the game and the final recommendation is conditioned by what type of gamer are you and what type of budget do you have.
If you are an avid gamer then the i5
is your partner, this processor will
run games smoothly for years to come.
If you have a tight budget and are
not so keen on spending your time
gaming then the i3 is for you.
It's up to you what you choose, but with medium settings both processor hold. Also as an additional information GTA IV is one of the games that is not highly optimized so any excessive use of settings will severely punish your FPS see behardware benchmark.