The answer to your question is yes you can connect it to a 450 watt power supply. However, there is a distinct possibility you will experience problems if you are overclocking and/or playing intense games. Personally I would recommend the Corsair VX550. It is a heavy duty unit that can easily deliver its' rated power.
Here are the official power requirements for the new ATI Radeon HD 5850 and HD 5870 video cards.
ATI Radeon™ HD5850 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four 75 watt, 6-pin, PCI Express® power connectors for ATI CrossFireX™ technology in dual mode.
ATI Radeon™ HD5870 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard.
500 Watt or greater power supply with two, 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four, 75 watt, 6-pin, PCI Express® connectors for ATI CrossFireX™ technology in dual mode.
The power supply recommendations are for an entire pc system.