I don't think your cpu will work with that motherboard, the 2800+ is socket 754 and the asrock dual sata2 is socket 939, altlhough on that board I think you can buy an add in board that has socket 754 on it for some $$.
Yeah it'll work with a 120gb drive.
And ddr sdram should work, what speed do you have though, pc3200 is what you'd want, but pc 2700, pc2100, would work, it'll just be slower.