I don't think anyone around here is going to be love with that Xion PSU. Can you get us the actual model number?
But the HD 5850 is one of those rare video cards that uses less power than a video card it replaces.
One issue for older budget PSU is they
lose the ability over time to output it's full rated power.
But I think it's safe to go ahead and upgrade. Don't overclock right away - give the new parts a chance to settle in and then do some stress testing (gaming is one way) at stock before testing more as you start overclocking. Compare HD 4850 and HD 5850 power consumption