i would suggest to test the RAM sticks in another computer, if you have another one with compatible RAM slots. that might be problematic, if you don't have another computer, and a friend who do have one with the same/compatible RAM slots might say no because he worry about his computer not working well after testing your RAM in his computer...
I didn't exactly do that, but: I got some more DDR2 RAM (exact same stuff, actually, G. Skill DDR2-800 "HZ"). I put it in the XBX2 and had the same results: either a "no memory" beep code, POST failure, or lockup during memtest.
The same (new) memory went into a different motherboard and had no memtest errors in 24 hours.
I RMA'ed the memory anyway. I'm also trying to RMA the motherboard, but Intel is telling me I need to test with memory from their tested system memory list. I don't really want to buy more RAM, nor do I know anyone with memory from that list. Should I just lie and tell them I borrowed a friend's memory and had the same problems?
Also, FWIW, with both the old and the new memory, I tried all different combinations of one or two DIMMs and all different slot combinations.