also the 6850 is a PCI-E 2.1 does that make a difference to whether it will fit or not
after doin some searchin it says ill need a minimun of 500w PSU
PCI-E is backwards compatible, but some PCI-E 2.1 card don't work with PCI-E 1.x. Mine did, but your may not, so I would suggest getting a PCI-E 2.0 card. Maybe a GTX 460 1gb because the performance is on par with the hd6850 and it uses PCI-E 2.0. Also, what is the name of your motherboard?
Also, you don't need a 500w. 430w is just fine. They say 500w because many "500w" PSUs actually only support 300w on the 12v rail, which is what the GPU runs off of. That 430w is a solid 430w and can support the rated power. It's solid.