If you're going to fill up all four DIMM slots, all four sticks should match each other exactly (put it this way, they should all be identical in every single minute detail) for the most reliable operation. Even a slight mismatch may result in your system failing to even POST or boot past the BIOS screen. And some systems will drop your memory speed (e.g. from DDR2-533 to DDR2-400) if you load in more than two sticks of memory.
With all that said, those particular Kingston memory should work - if you remove your existing modules from your system. That's because the OEM memory which came with your Dell system does not mix well at all with other memory modules besides what you can order directly from Dell's parts department at inflated prices. Also, all Kingston ValueRAM modules are unbuffered (note that registered = buffered, and your system's memory controller does not support registered/buffered memory).