HD 4870 has 1.2 teraflops computational power as seen
here on amd's website.
The PS3's gpu has 1.8 teraflops computational power (I never said it was all used effectively) as seen
here along with many other sites that did reviews on the PS3 when it came out.
Finally, the PS3 has 50% more processing power than an HD 4870; this is just simple math (1.8/1.2 = 1.5 = 150%, meaning a 50% increase).