Problem with newly assembled system

Your system doesnt have enough power.
9800GX2 requires a Minimum of 580W from you PSU not to mention at least 40 amps on the 12 volt rail.
(remember the gx2 is technically like having sli so imagine dual 9800s and the power requirements)

Random restarts and reboots are almost always a psu problem.

basically whats happening is when you take out your sound card your giving it barely enough power to startup, once in windows the card begins to heat up and once it gets warm enough and the fans kick up it overloads your psus peak power and shuts down. this is why your system is having trouble. get a 700 or better watt psu and you should be fine.

At idle your Q9450 and 9800GX2 are probably using less then 200 watts so I dont think its your power supply.

Have you tried using just 1 stick of RAM at a time? Rotate the sticks till you have tested both in both the first slot and 3rd slot of the motherboard.

I do admit i was going by their recommendations on power but it could still be a power issue just maybe not a watt based one.

-also you might want to try these

a. disconnect all peripherials such as cd drives and hard drives and take out all but 1 stick of ram, your sound card and start the system. see if it gives you any sort of beep codes or if it boots ok.

b. if not try a cmos flash find the jumper labeled j5 and while the system is off short it out for about 20 seconds and then take it back off and restart your computer.

You need to locate what part is causing the problems. since you said the only differences is the motherboard and ram start with those. which leaves you with ram at this moment. test the ram sticks 1 at a time.

if its memory related your memory might be being misconfigured by the bios when reading its spd. from what your telling us it can really only be 3 things. the ram, the motherboard or the power supply. (did the other system use this processor as well ?)

(btw what version of windows are you running. if its xp and your not getting a bsod it probably is still the psu. thats a big red flag if your not getting a bsod. if its windows vista this is normal because by default vista will restart automatically and not normally show you a bsod.)

until you test the ram and psu and get back to me there are still a couple other more in depth problems it could be like the ram configuration problems. it is overclocked