only interest is in keeping the power as low as necessary.
That is determined more by your components than anything else.
A more efficient power supply will have a lower draw from the wall, but we're talking small percentages.
Any good quality 500-550 watt unit would be more than enough for your system, I like these
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 5000 series cards which use less power due to their energy efficiency. On the other hand the top of the line 5970 video card uses more power.
A high quality 500 to 550 watt psu will have a +12 volt rail rated at 40 amps. A high quality 700 to 750 watt psu will have a +12 volt rail rated at 60 amps.
In addition the power supply should be at least 80+ Bronze certified for energy efficiency. There are some models available which have achieved 80+ Silver and 80+ Gold Certifications.
Before purchasing a new psu you will need to decide whether you will eventually have a pc with one or two video cards.
Corsair and Seasonic are two 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. Some of the newer models come with a 7 year warranty. Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts and the Antec TruePower New series which are major improvements over Antec’s older psu’s like the Basiq models.
Yes it should be more then enough to run it. That unit has 30 Amps which is more then most cheaper 550 watt PSU's. Although you arent gonna see too much of a difference between those two cards the 4850 will perform a little quicker but the 8800GT is still a great card. If you are looking for an upgrade at least get an HD 5770
Now if you are thinking of maybe getting a little better of an upgrade check this SeaSonic.
I'm sure it's still quieter than my current PSU, a 600W Jeantech JN-600-AP.
The fan is stops working once in a while and overheats my system ending in shutdown; the main reason I'm looking for a better PSU.
Regarding my 8800GT; it's 512MB so the 4850 would be something of an upgrade - it's £65 and the 5770 is double that. I'm only looking for an increase of around 10fps as a couple of the games I play can sometimes get under 60 on my resolution.
That SeaSonic looks amazing though - I'll have a look around. Trying to keep the electricity bill down