The chipset is not the motherboard, only a part of it. The chipset is composed of the southbridge and northbridge. The southbridge communicates between the processor and the PCI bus, the SMBus, a clock, CMOS, and most integrated peripherals. The northbridge manages communications between the processor and the RAM and graphics card (but, more specifically, the graphics card's port [AGP or PCI]).
The speed of the RAM is most important for multi-tasking (using multiple programs at once). When you are opening an application, your processor will search for the app's instructions on how to execute and run the app. If those instructions are not in the memory(RAM) at that time, they will be transfered from the hard drive (the faster your memory is, the faster this process occurs). When the hard drive must be accessed in order to do this, it is called "loading". When you are multi-tasking, the memory will send what components of the program you are not using back to the hard drive and put it into, what's called, virtual memory. When you use a part of the program that you weren't before, the memory accesses the virtual memory to retrieve that component. This process is called "swapping" (and, again, the faster your memory is, the faster this process occurs).
I'm assuming that you still have the stock motherboard in that pc (usually when you buy a pre-assembled desktop, they do not tell you what motherboard you computer has). If so, you should get two 1GB memory sticks running at 800MHz because I highly doubt that that stock motherboard has four slots for RAM (I could be wrong, but with pre-assembled desktops, there are usually just two slots). You won't really need 4GB of RAM, so just get two of the RAM sticks you found (instead of four).
DDR2 is just the type of RAM, the most common type in existence.
I was wondering wether different speeds of ram really matter.
I own a HP M7580 with 1gb (2x512mb) ddr2-sdram.
It has an Intel