The 5670 would be quieter and run cooler. It's also got several nice features. However, it's a little weak for gaming, but that shouldn't matter too much at that resolution. The 4850 would be more powerful, but slightly louder and less future proof. It would still be good for gaming with max details (or very high details) if the monitor was upgraded to a 1600x model, and could be playable with lowered details on a 1900x monitor.
If he wants to expand later on though, instead of 2x1GB, you could get 1x2GB, and add another 2GB stick later, they are priced quite similarly, although it won't be running in dual channel - the slight performance loss wouldn't matter though IMO.