First off welcome to the forums. Please read the following post to better understand your ram and an amd processor.
http://forum.giga-byte.co.uk/index.php/topic,2515.0.htmlKudos to absic for putting that thread together.
now to the next. 7-8-7-24- 2n is what they are rated at that being said download cpud and it tells you where your timings are which is which. So you have 4 x 2 gig sticks id put the 2 in and boot it then run memtest on them for at least 10 passes if they pass take them out and swap them with the other two sticks you left out. Run memtest another 10 passes. Please report your results back so we can further diag your problem.
Could be bad sticks, you could have them set to ganged in bios instead of unganged, ( cpud will also tell you this ). Make sure voltage is set to 1.6 default is 1.5 usually. You can try upping the voltage a little at a time to see if you need to take it all the way to 1.6.
Might be you need to set timings to 9-9-9-24 that seems a good speed for 1600 but then again if you run them at 1333 at 7-8-7-24 you will more than likely get as good or better performance because your timings are lower.
Alot of variable like i said report back with the info also what the rest of yor build is cpu, type hard drive and so on please and again welcome.