Hi, I had the same problem and have a solution. In the bios there is a redundant memory slot setting.  By default it is set to disabled.  Setting it to enable allowed the server to boot properly and make all 4 slots available to the OS.   I am able to run Windows Server 2008 (x86) Enterprise with 8GB of ram.
Has anyone else seen a situation where a PowerEdge 1750 refuses to accept a full 8GB of RAM? I have a system that's been fully upgraded to BIOS level A12 (latest), and in which everything else works correctly. It started off at 2GB (4 512k sticks), and I swapped the first bank for 2 GB sticks to make 5GB. That worked fine, but then swapping bank 2 to 2x2GB (maxing out the machine at 8GB) causes the machine to reset itself as soon as it completes spinning up the SCSI disks. I've tried several different sets of modules, all Registered/ECC and the correct spec, but none work. There appears to be nothing wrong with bank 2, since I can put the 2GB modules in there along with 512k in bank 1, and the system boots. It's just that it fails at 8GB (Windows 2008 32 bit is installed). I heard from one source that setting the integrated USB to non BIOS support might help, but that didn't work. Any ideas appreciated.
Memory issue with old PE 1750