Both Olympus SP-590 UZ and Pentax K-x have 12.0 MP resolution sensors but Olympus SP-590 UZ's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Pentax K-x's sensor is APS-C (23.6 x 15.8 mm ).
Since Pentax K-x's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Olympus SP-590 UZ.
Below you can see the SP-590 UZ and K-x sensor size comparison.
As seen above, Pentax K-x has a 13.5x Larger sensor area than Olympus SP-590 UZ. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.