I have Vista 64bit installed with 4GB ram on my Gigabyte GA-X38-DQ6. Everything working well.

I just tried to instal another 4gigabites for a total of 8GB ram on 4 2gb sticks. The ram is identical.

Windows Vista will not boot - shows blue screen for a few seconds, then restarts.

In bios it shows 8gbs properly. Whats the deal?

Try underclocking it and boost ram voltage and see if it helps.

^Agreed.

Ive never set up any overclocking befrore - unless the MB comes as default over clocked some? Here are my memory specs.

Model
Brand mushkin
Model 996587
Type 240-Pin DDR2 SDRAM
Tech Spec
Capacity 4GB (2 x 2GB)
Speed DDR2 800 (PC2 6400)
Cas Latency 5
Timing 5-4-4-12
Voltage 1.8V
Heat Spreader Yes

Try 2/2.1 volts not the stock 1.8 volts.

You could have gotten a bad stick of RAM?

Go into the bios. Set your ram speed manually. Increase your ram voltage as others have suggested to 2.1V or 2.2V. I am running the same board with an E8400 and 8 gigs of Gskill PC1000 and not a single problem. I've got the FSB up to 445Mhz and the ram is around 1066Mhz. CPU clock is 4.0Ghz

As i mentioned, I'm not an over clocker - i just need stability overall.

Here are my Settings under Motherbord Intelligent Tweaker....

Standard Timing control - i have it on the [ auto ].
Cas Latency Time = 5 / Auto
DRAM RAS# to CAS# Delay = 5 / Auto
DRAM RAS # Precharge = 5 / Auto
Precharge delay (tRAS) = 18 / Auto

However my memory have a 5-4-4-12 Timing so should i have those numbers correspond?
I dont really understand over clocking very well so i dont mess with it - altho i would like to learn what im doing some day. But my work needs stablity, and i dont have time to mess around with tweaks that might cause crashes.

In order to stabize your ram, you need to increase to voltage. Go into your biod and increase your rams voltage. Start with 2.0v then try 2.1v. I wouldn't go past 2.2v

OK, I got it to boot now.

I went into the MB Intelligent Tweaker (M.I.T.)
Under "Performance Enhance" (was on Turbo) I set it to Standard. I Left every other setting alone and it boots now to Vista 64 and displays 8GB of Ram Properly.

...But still, i am wondering if all the Auto settings i have the MB on are in my best interest? Should i still try increasing the voltage if the system is working now? Will increasing it to 2.0v or 2.1v allow me to work with "Turbo" enabled again? - or even the "Extreme" setting - which apparently (lets the system operate at its best performance level). Turbo is default it says in manual.

-There are so many voltages here - I asume its the one that reads "DDR2 Overvoltage control"? Right? Im not taking any assumptions lightly. One mistake and im out of a job.

learn to overclock youslelf... its way better than letting the bios do everything

Rule 1: Never use any automatic overclocking feature, either hardware or software based. They will mess things up. Either overclock manually or don't oc at all.

you shouldnot use any automatic overlocking feature showed on cmos, its able to be worse, usually motherboard info to upgrade Ram is not true , also donot increase voltage.
u should use the same manufacture's name, same bus speed, and up Ram lower manufacture infomation
and to make sure the new ones work well, u remove the old Ram, work only new Ram, if its still problem, i think u know what u have to do next.
have a lot of ways from this issuse if u have more information i can help u.

Agreed, that turbo crap is just that, crap. If you are really concerned about stability for work and it's stable, then leave the damn thing alone. If you want to OC then do a little research before hand. Make sure you write down the settings before you start so you can put them back if you can't get a stable OC.

So then i should adjust these values bellow....

Cas Latency Time = 5 / Auto
DRAM RAS# to CAS# Delay = 5 / Auto
DRAM RAS # Precharge = 5 / Auto
Precharge delay (tRAS) = 18 / Auto

to this...?

Cas Latency Time = 5 / Manuel
DRAM RAS# to CAS# Delay = 4 / Manuel
DRAM RAS # Precharge = 4 / Manuel
Precharge delay (tRAS) = 12 / Manuel

Since 5-4-4-12 is what it says on the sticks. Right?

Yes, you could tighten up the timings, but it won't make a huge difference in real world performance with the Core2 Duo/quad. If it gets flaky just put them back to auto.