Is it possible to replace the motherboard?

Of course, a PC is like a lego, just be careful, not all the components work with each other. I advice you newegg.com you can find all the pieces with info on compatibility and much more.

You can even replace the motherboard with the same model. The 530 supports two different motherboards: G33M02 and the DG33M03; the G33M02 uses Core2Duo CPU chips from Intel (not all, just some) while the DG33M03 uses Core2Duo or Core2Quad chips from Intel (not all, just a select few). That would allow you to speed up your system if you install a faster, compatible CPU chip or even swap out a G33M02 with a DG33M03 motherboard.

If you install a motherboard from newegg or elsewhere, you may need to remove and/or replace the backplate where the USB ports, the video, ethernet and sound port exit the case.