Laptop RAM sticks are more expensive because of the vastly superior architecture design required to maintain their compact stature. Fitting large amounts of RAM into a stick thats usually half the size of a normal RAM stick is a difficult task indeed.
And yes it does matter what motherboard you use, as the CPU socket types vary and so does the expansion slot bus types (I.E. your graphics slots like AGP vs PCIx and the variations there of). Matching of CPU socket/Motherboard cpu socket, and aforementioned graphics slots is nessecary. The video card I linked is PCIx x16 2.0 bus type, but runs just as strong on the Original PCIx x16. The physical slot is identical.
The motherboard I linked is an AM2 socket type, likewise the CPU is AM2 socket type, etc etc.
Also, if you buy the items I linked, please note that the power connector on the graphics card is 6 pin, and the power supply I linked only has the typical 4-pin Molex connectors. The Video card comes with an adaptor for this, to make the 4-pins work with the 6pin on the video card, it'd be in the box. Also note that the SATA HD power connector is not a 4-pin, but the powersupply comes with an adaptor to make the 4-pin work with the SATA connector.
And your welcome