=[...I wouldn't exactly say that an 5770 is crap...I have one and it's OK...but yeah it's not futureproof...depends on your max res.
As for power, i5 OC + HD5850 = 550-600W minimum IMO. Even without the HD5850, at 450W, you're skating on very very thin ice. Great PSU wattage calculator:http://extreme.outervision.com/psucalculatorlite.jsp
...I always add an extra 50W jic