So you tried both new 256MB sticks of RAM with the original single 128MB stick that came in the DELL, and they both work right? The commensurate 384MB is shown and the system boots and windows loads. So the new RAM isn't faulty.
Equally it's not the power supply. RAM isn't a huge power drain, and adding a greater amount is not going to overtask even the most marginal PSU. If it did, the PSU would immediately fail to provide enough power to spin even a CD, never mind a larger capacity stick of RAM!
So what's to causing the problem? You've actually narrowed this down quite well. The PC ''needs'' the DELL 128MB stick of RAM to boot...but what's so special about it? Must be something like it's stick of RAM with parity checking (an extra chip (don't count them, it's no real indicator, or help!) that checks basic data input/output function) which typically for a DELL means that once again you can't just install generic replacment parts for a quick and easy upgrade path. As soon as remove that stick of RAM, the parity check is gone, and so too is the working system.
RAM makers sell many types, both with and without parity checking, for just this particular reason. It might be possible to enter the DELL BIOS and disable the parity check option (PC's here all have some BIOS entry regarding parity on/off, but happily none are DELL. ;D ), but again it's a DELL, which means they'll have their own very limited BIOS options (''we must stop users having control over their hardware!'') which doesn't allow you control over that.
Good luck though,
I have a Dell Dimension 4100 with 1G processor(service tag 6LZH701). It originally came with 128MB of PC133 SDRAM. I added another 128 a while back and was happy with the performance under WinMe. Now with XP I am slowing up and want to add two 256 MB chips to max out the Mobo according to several sources. I bought the 256 chips and tested each individually with the original 128 and they worked fine and were recognized as 384 during boot. So I logically took out the 128 and added the second 256 and tried to boot.
Nothing happened. The screen stayed blank and the fans spooled up along with the hard drive, but it never kept chattering like it was booting. I reversed backed to the 128/256 setup and still nothing. Panic started to set in. I pulled the other 256 and booted with just the original 128 and I managed to get it to boot. I put the 256 back in with the 128 and it booted again. 256/256 and again nothing. So I back tracked and evaluated my position.
I thought maybe the power supply isn't strong enough so I pulled out my dinosaur modem and unplugged my secondary CD/DVD-ROM and tried the boot process all over again. Still won't deal with the 256/256 chip setup.
Any insight into this problem would be most appreciated.