Will a too hight psu kill you pc?

You might end up with slightly lower efficiency, other than that your components will only draw the power they need.

Not to worry. A psu will draw only the power that is demanded of it. A psu operates at it's best efficiency in the middle third of it's range.

If it operates at maximum capacity, the cooling fan will spin up and become noisy. On lower quality units, an underpowered psu may not deliver clean power.

If a psu is much stronger than needed, it will be a little less efficient, and you will have paid more than you needed to.

so if i get one all it will do is slow my pc?

NO NO. A too strong PSU will run just as well as a properly sized one. It has no effect on how fast or slow your PC will work.

on the web site i am looking at the cool master 700w with pc case cost less than getting a pccase and a 650PSU

OR A 600W

if i have a too strong PSU will the life of the psu be short?

The quality of a PSU is very important.
Good quality PSU's come from Corsair, Seasonic, PC P&C, Antec to name a few.

The only vendor of cases that I would buy a PSU from is Antec.

Here are a couple of links to lists of PSU quality:
http://www.eggxpert.com/forums/thread/323050.aspx
http://www.overclock.net/power-supplies/154057-jonnygur...

Look to tier 1 or 2 only.

If a good psu has sufficient 6 /8 pin pci-e connectors for your graphic/s cards then it should be OK.
The wattage of a PSU is not as important as the amps it can deliver on the 12v rails.

No.

THANKS for your help you the best  

the case with the psu i was looking at was http://www.ebuyer.com/product/177749

Coolermaster? The company is known for very good pc cases. Unfortunately the same cannot be said of their power supplies. Their psu's are not stellar performers.

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.

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 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. 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 Truepower New series which is a major improvement over Antec’s older psu’s like the Basiq models.