Acer Aspire5315 memory upgrade problems

Hello everyone, Im Rob, im new here.
I have an acer aspire 5315, the specs are
intel celeron 550(2Ghz,533 Mhz FSB 1 Mb L2 cache)
1Gb DDR2
80 gb HDD with vista basic preloaded
DVD dl writer

I got this free with signing up for talktalk broadband. I didnt like the glitchyness of vista so it was seldom used. I won a competition where the prize was a copy of windows 7 ultimate. I decided i would upgrade the acer as the microsoft checker said it would run on it and not on my older xp medion. I did a clean install on a new 250gb hard drive of the 32bit version and although it runs 100% better and faster than vista on 1gb of ram I still wanted to upgrade to 2gb.
When I opened up the back I was expecting a pair of 512 Mb modules but there was 1 single samsung 1gb 2Rx8 PC2-5300s-555-12-E3, there were the 2 slots but only one module, ok I thought, so i took it out and went memory shopping. I managed to get another identical samsung 1gb and installed it into the other slot and put the original back in also. It would not boot not even to the Acer, preparing to boot OS screen, just the power light comes on, the fan whirrs the cd drive spins as does the HD but nothing comes up so i put the original HD back in with the original preloaded vista and no difference still just a blank screen.
I then tried swapping the modules over in the slots and got the same with both hard drives.
In the end the only way to boot up was with only 1 single 1gb module(both the modules worked so its not faulty memory), Dont think anything on the motherboard is damaged as with only 1 module installed it boots and runs perfectly well(vista runs as well as it can, windows 7 perfectly).
So I am I overlooking something or doing something wrong, or would I be able to put a single 2gb module in???
Sorry about the essay folks but wanted to give all the info possible, any help and advice will be very gratefully recieved

 

place your real memory in bank 0 do a memtest
place your real memory in bank 1 do a memtest

no errors --> your new memory is broken

 

Thanks Dython I have tried as you suggested....
Both work in the top memory bank but nothing works in the bottom one so it appears the bottom bank/slot is the problem. Is it easilly fixable??, or should I go for a 2gb module in the top slot?