You say it can be upgraded at a later date, but you don't know what psu is in the system? If it's just enough for a 4850 then unfortunately it's going to be no good for a 5850/5870, or for that matter more or less any video card that's better than a 4850. Like I also said in my previous post, if it's some cheapy generic mainboard then you will struggle to overclock your cpu which will hold back (bottleneck) any future video card upgrades. I know GTA 4 loves quads and the higher the clock speed the better. Seriously if you're going to blow 800 quid on a new machine at least do a little bit of homework before wasting your money, surely you want best value for your hard earned cash?
BTW If it's purely for gaming then I'd probably suggest going down the core i5 route and spend more of your budget on the video card.
If you tell us what size monitor you have, we can then give you a rough idea of how well games will run on it.