The 5850 is rated as having a max TDP of about 150W, which is only slightly higher than the GTS 250 you already have in there. The Cooler Master Extreme 460W unit isnt a good quality unit, and i would have recommended getting a different unit originally that would have allowed for some more upgrade room but it should be able to handle a 5850 fine, just dont run furmark and prime95 to stress test it or you may push it too hard.
You'd have to get a 2 x 4-pin molex to 8-pin PCI-e adapter. There is still no guarantee it'll work. The 6-pin connection you already have needs to deliver 75W and the 8-pin 150W according to the specs for the video card.
the video card and whatever you have running that takes 75 watts are not the only items in your computer that require power. Everything from the ram, drives, etc that is in your system will use some power even when idle. WHen running at speed your computer may have all the ram dimms, the processor and accompanying fans, the gpu and its fans, your cd/dvd drive the ethernet connector and who knows what all else running. All of these take power and you only ahve a limited ammount on a 460w psu not to mention the fact you need to connect all pwoer connectors on your card to properly power the gpu and all its fans so it does not overheat. http://www.antec.outervision.com/ try using a calculator like this and plug in what parts you have connected laong with all the details of your computer and it will better explain what your power needs are.