I was thinking of getting a HD 6850, On AMD's Site it says "500 Watt or greater power supply recommended" So i don't think i could get away with it. (I'm currently using a HD 6570 low profile)
No, a 6850 needs more power than the pci-e slot can deliver. Namely a 75w 6 pin pci-e power connector.
Using molex to 6 pin adapters will not do the job.
The strongest card you can use without a psu upgrade would be a 7750.