Welcome to SevenForums: You can download your manual from Asus site. Your board requires 184 pin DDR memory http://support.asus.com/download/dow...Language=en-us
If you read through the memory section it will tell you which slots to use. Page 1.7 / 1.7.1 System Memory Overview/ 1.7.2 will explain memory configuration and what slots to use. (When downloading the manuals I used china as the global site always has to much traffic to download and the other is P2P.
Also on the manual page( I used Vista 32bit for your OS search) since it is the closest to Windows Seven you can also download the manufactures A8N-VM CSM Memory QVL which is what the Asus manufacture has listed as qualified memory modules for your board http://support.asus.com/download/dow...Language=en-us
I believe your board uses the AMI bios so that would be the beep codes you would be looking for.
So yes it states Conventional Extended Memory Failure. Yet it will still boots with a different card so..
Since you have stated that you have put in a "no name video card" and it boots up fine I would suspect your 8400gs has probably died on you.
Their are a couple of things you can try as far as your video card goes.
1.Turn off your system, unplug it,then ground yourself by touching the side of the case before removing the card. Once it is removed use a can of compressed air to blow out the slot and the cards heatsink and fan.If your card requires additional power from your power supply make sure you plug it back in.
2. re-install the card reversing the above directions.
See if your system will boot.
If your system boots up then you still might want to test your memory using memtest86 to test your memory for any errors.
You can download it here http://www.memtest86.com/download.html
You will have to burn the ISO to a CD let it run at least 5-7 passes. This will check to see if you have a memory problem once that stick has run then remove it and put in your other stick and run the test on it.
I think it is good idea to download your manual and read through it, You will find what memory your boards manufacturer recommends and which slots to populate in order to run dual channel or just avoid a no boot situation due to not having your memory in the proper slots.
My real thinking is that your video card has just died on you since a different one booted up fine. Cards die their is no real rhyme or reason they just do.
You could try it in a friends system or if you have another system try it in it. If it won't boot then you've pretty much found your problem.
Hope this is helpful and not to confusing. Fabe