I have four modules of PC2-3200 DDR2-400 256 mb in my Dell 8400I think the only upgrade I will do in a year willadd memory. Is getting 2 512 gb modules and keeping2 modules of the 256 mb in there slots the the bestway? Thanks, Ken

Its fine to add two banks of 512. However when you do decide to keep in the other two banks, be sure that the timings from the new modules is exactly the same. In short,All 4 sticks have to exactly match Not doing so would in most cases result in unstable performance. Mixing memory is a big no-nobtw you mean 512 MB not GB :(

my CPU-Z is showing 3-3-3-8 and 4-3-3-8 on the SPD timings table,so maybe I already have two different timings in there.I have not had problems yet, but would like them the same. Ken

Consider yourself lucky then, it has and will always be a golden rule, do not mix memory timings.