After two frustrating days, I

After two frustrating days, I discovered there is a motherboard problem with my XPS 720c. Turns out with even new memory modules from Dell that slots 2 and 4 don't function properly. After installing two new 2Gig modules from Dell, everything reads 4Gig of RAM etc after hitting F2 on boot; however, SysInfo in Vista shows 4 Gig of RAM but then only 3Gig Physical memory. Ran memory diagnosis which showed problem after 10% of the first pass. When I put only one module into Slot 1 everything is ok. So, how do I deal with this? I've read online this mobo from Dell sucks and has a reputation for going bad. On top of it all, I have now purchased $200+ worth of Dell recommended memory from Dell that is worthless. I'm hoping Dell has a reasonable way to help an extremely loyal customer.