Pixelbator system requirements state that you will need at least 4 GB of RAM. The cheapest graphics card you can play it on is an ATI Radeon HD 2900 XT. To play Pixelbator you will need a minimum CPU equivalent to an Intel Core 2 Duo T7250.
Pixelbator will run on PC system with Windows XP SP3 (32/64 bit) or later and upwards. Additionally it has Mac and Linux versions.