I had a few Dell Optiplex GX270 PCs on my work with 256 mb memory on board, so they were quite slow with Windows XP. After I replaced the memory with 2 x 512 mb modules in dual channel mode, they become much faster, however one box started to hung, turning the monitor into the sleep mode. It works fine with one module installed and I've tried different combinations of modules, so it's probably not connected to modules itself. I guess it has something to do with the dual channel not working properly. If there's anything I can do to troubleshoot this issue? Thanks!
Could be a bad motherboard. Were you taking precautions against static electricity, such as wearing a ground strap, or keeping on hand on the computer's chasis?
You could try a memory test from EBCD. If it fails with completely different RAM modules, then I would think it's a bad motherboard. If the memory test passes, then I'd run chkdsk and make sure the paging file is set to a large fixed size, say 2 GB.