Memory problem

I have an Inspiron 8500, and I wanted to upgrade to VISTA.  When the software started testing my system, I got the message that I need 512MB of memory.  My system came with two 256MB sticks that are installed in slots A & B.  Not having enough sense to check my system memory, I instead went out and bought some more memory assuming one of the sticks of memory had gone bad.  When I installed the new memory (2 512MB), I still only had 512MB of memory.  After removing and switching the memory in every combination possible I discovered that the system would not boot with memory only in slot A.  I did the Dell current configuration and it says that I have 512MB in slot a, but nothing in slot be.

All the memory sticks work if installed in slot B; anyone have any ideas.

Thanks!