I am gettin a new graphics card and im goin 4 the sapphire amd radeon 7850 but my psu is only a 350w one so does anyone no were I could get a gd 750w one to fit in my asus cm6340 pc or how I could find out were!
A 500w PSU would be fine for the HD 7850. However, if you have an AMD CPU then you should get a 600w PSU instead since they consume more power than Intel CPUs. Which PSU depends on how much you want to spend.
In general I recommend PSUs manufactured by Seasonic. That includes Seasonic, XFX (all of them), and Corsair (some models). Seasonic's PSU generally do no go on sale or are discounted as much are other brands so they can be relatively expensive, but at least in my opinion no one can match them in overall quality.
Corsair PSU are a popular choice because their overall quality is pretty good and due to frequent discounts or sales, they are relatively inexpensive. Their CPUs are manufactured by Seasonic and Channelwell Techologies (CWT). While CWT not considered as good a Seasonic, they are still considered one of the better PSU manufacturers around.
A 500w PSU would be sufficient. Of course buying a PSU with higher output than 500w means better upgrade options. Like dropping in another Radeon HD 7850 for some Crossfire action. Or a single powerful like the GTX 780.