True, desktops are much cheaper due to the fact all parts don't have to be custom made as such; you can quite literally shove parts in and make a computer. But, if you're set on a laptop, then go for a strong quad-cored CPU as GTA IV seems to like those. You'll also ideally want a strong graphics card, which, along with the usual 6/8GB RAM and stuff, on a laptop, will set you back easily a thousand USD. I won't suggest one as I don't know your price range, though!