lets try to pinpoint where the problem is coming from. i assusme you just built this computer recently? i do not think manufactuerers like Dell, compaq, e-machines would make a computer with a gigabyte motherboard and SATA.
assuming you built this machine yourself, did you always have this problem or did it start up at sometime. most importantly, after this happens you should look in your bios to check your temps. if you are overheating than you would know that it is a cooling issue.
it really does sound like a hardware problem to me, and you could be right about the memory, what is the brand of the memory you have for your pc3200? and you have 4 identical 512MB modules correct or are they different brands?