I'd try a memory test program first. Something like
memtest86. Use each stick by itself individually in the system and let the program run through several times. When one seems to pass, install one of the others and rerun the test again.
Just a thought here, but it might be a motherboard problem too. I don't know why for sure, but some motherboards don't like having all the memory slots maxxed out (memory in all of them). It's rare, but I have seen it happen that a system will get errors with all the memory slots having memory in them. Check each stick with the memory tester first though before looking into this possibility.
As for mixing it with the Kingston, you'd just have to try it to see if they'll work together. Sometimes different brands of memory don't like to work together and they say to try and get sticks as close to each others properties as possible (brand, size, etc). One more thing, if that Kingston isn't DDR400, your memory will all slow down to run at the speed of the slower stick of Kingston (like if it's DDR333). So if the Kingston is alot slower you might be further ahead leaving it out.