First, lets take a look at the official ATI system requirements for the HD Radeon 4890 video card:
500 Watt or greater power supply with two 75 watt, 6-pin PCI Express® power connectors recommended
600 Watt or greater power supply with four 75 watt, 6-pin PCI Express® power connectors for ATI CrossFireX™ technology in dual mode
Since you only have one video card a high quality 500 to 550 watt power supply will be sufficient. An appropriate choice would be the Corsair VX 550. It can easily power an overclocked system with any single video card made. Corsair has a reputation for high quality power supplies. They consistently earn high marks in technical reviews. They are very stable, reliable, and come with a 5 year warranty.
The OCZ ModXStream Pro 700 watt power supply is actually more than you need for an overclocked system with one video card. In addition there is another problem. OCZ incorrectly advertised the maximum operating temperature as 50C. The problem was discovered during technical reviews. The psu was actually rated at 40C. OCZ was forced to change their advertising. That happened last March. Despite the issue it is still a good power supply.