What PSU will i need for 2X 9800GT?

thinking about SLI with my Nvidia 9800GT, but have a 550 watt psu and a non-compatable MOBO. my MOBO was only £40 so i am planning to buy a New one, with SLI compatiblitly. i know my PSU isn't up to scratch, so how big of PSU should i get. also which Manufacture is the best for buying psu's?

i was thinking about buying a 600Watt PSU.
Here the rest of my specs:

Amd Athlon BE 2.7DC
250GB Maxtor HDD
DVD drive
Belkin 9000Wifi card.

so how many WATTAGE will i need and what make is the best. also i only have a budget of £70

The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily power a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the 12 volt rail(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.

Corsair, PC Power & Cooling, and Seasonic are some of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Lately we've been seeing a few other brands offering some high quality units.