Thanks for all this help. Is it thought that the wrong RAM is causing my problem please - Yes or no? In which case, I better source the correct memory modules. YES! You bought RAM that's not compatible with the XPS 8900. And yes, you better source the correct RAM...
Hi, Trying to fit two new Ram modules to XPS 8900. Upgrade from 16 to 32GB This proved impossible without removing GeForce bracket. So I did this - ensured all modules were securely seated, reinstalled bracket. But the pc will not boot - it runs, but black screen. Have spent hours on this now - at the end of my tether - no idea what to do next. Help appreciated. Thanks 
XPS 8900, ram update problem