|"i've test my external memory and it's working fine"|
How did you test it? Has it EVER worked in this machine before? If it's new RAM, did you make 100% sure it's compatible with your system before you bought it?
And what about this 'built in' RAM...how much is built in? 32MB, 64MB, 128MB, more than that? Is it used by the system or just the graphics? Did you try booting without the removeable RAM installed?