This is because of your power supply. the 5670 performs between the 4670 and the 4770 but uses such a small amount of power that it does not need a separate PCI-e power connector, powering itself from the PCI-e bus.
Of the two cards you presented, the 4850 is significantly better. The problem is, your PSU will not handle either card. The best I would use with that PSU is a GT240. A Radeon 4670, while cheaper and likely to preform as good on your computer due to your CPU, uses a little more power. The Radeon 5670 also uses a bit more power than the GT240 and would be pushing it.
So you guys are saying the HD 5670 is better than the HD 4850, and I don't need to buy a new PSU?
Oh and by the way my current card is a http://canadacomputers.com/index.php?do=ShowProduct&cmd... and it draws 211 watts at load!!! So hopefully the HD 4850 has less power consumption so I don't need to buy a new PSU.