a right way to place the DIMMs for optimal performance and it doesn't work, you're stuck.
Check your system manual. If you get the 2-1GB DIMMs working in say channel "A" ... you should be able to see 'Dual Channel' or 128bit Memory Operation during bootup. Once you get your system running in the dual channel mode...then add the other pair of 512MB DIMMs. If it doesn't work ...then it doesn't work and you'll have to remove that and accept the limitation. That's why I always recommend getting all the RAM you expect to install when you buy the system as adding RAM later can be tricky.
Let us know how it turns out.
I received a Crucial 2GB Kit to upgrade the memory in an old Gateway GT5432. I've installed the 2 new modules in the 2 slots the original two modules occupied for 1G, and that works fine. There are two more slots available, but when I insert the 2 original modules to try and increase the memory to a 3G total (2G Crucial upgrade and !G original memory)the machine will not boot. The unit will display a Gateway screen with an F2 or F10 option for BIOS or "something", but neither key will advance the process and ignoring the keys does not advance off the screen either.
I guess I'm trying to find out if/how I can utilize the original 1G of memory on this machine. I've tried all four modules in either set of slots to no avail.