a. enter the crucial site and pick your new motherboard from their list - it will tell you the type of memory to get. basically ddr2 started with a fairly low speed 4300 then improved to 5300 then 6300 etc. the faster speed the faster the memory works.
b. you decide which graphic card you want and then look at its specifications. Some require a separate power supply others just plug into the pci-express slot.