well, i was asking because memory is not going to be the basis in which you want to buy a computer. The CPU is the most important part as it serves as the brains of the PC.
For Example, For Intel CPU's, Core i7 is the best, then Core i5, then Core i3, then Core 2 Quad, then Core 2 Duo.
Core i3/i5/i7 can ONLY use DDR3, Core 2 Duo/Quad can use either DDR2 or DDR3 (depending on the motherboard) but those CPU's will not fully utilize DDR3 so there really is no point in using DDR3 with Core 2 Duo/Quad.
List what CPU's the that the HP/DELL/Gateway PC's have, most likely, you will be limited to either DDR2 or DDR3, you will not have a choice.
Also, is this a pre-built PC or is it something you are configuring yourself?