Bad Ram Slot on Motherboard?

Hi, this is a follow up question from this one:
https://www.experts-exchange.com/questions/26010032/BAD-RAM-Memory-modules.html

In my previous question i was worried that my Ram sticks where Bad, faulty, but i runned some tests on each Ram stick at once (separatedly), and they all came OK, with both Dell Diagnostics and MEMTEST 86+.  

The strange thing was the following:
I formatted my harddrive, and reinstalled XP Pro Sp3, after that i runned again MEMTEST 86+ with all 4 sticks inside, and all Tests where OK!
But with Dell Diagnostics, The tests only passes when ONLY ONE stick inside, if i put one more on the SECOND SLOT, All memmory tests have ERRORS (infinte number of errors!)

So my Questions are:

1) As someone else suggested (nobus) maybe is my motherboard, so How Can i Check if is actually my motherboard?? (All Dell Diagnostics tests on motherboard are OK).

2) If All memory tests on Dell Diagnostics throw errors, why do my Programs work (Ram demanding programs like Photoshop, Autodesk  Maya, Adobe After Effects....) ???

Thanks for your help

My Specs:
Dell Precision workstation 470 (3 1/2 years old PC)
XP Pro Sp3
1 Harddrive SATA  160 Gb (or 150 Gb) approximately
3GB Ram Total: (400-MHz DDR2 SDRAM)
      2 *  512Mb Ram Modules
      2 *  1Gb Ram Modules