fixing shared buffers

[zedjr]

Every time i set shared buffers to over 1024mb in postgres conf file when i restart os and postgres service it doesnt load and i have to always sent it down to no higher than 1024mb. I have effective cache i think set at 4048mb and i thought this owuld fix it but somethign else is causing this i guess. so how do i fix this where i can set it higher as i should be able to set it at least to 2 or 4 gb. i have 12gigs of ddr3 1600 on x58 board with 8.4 installed but same thing happened on all my systems before with 8.3 also. Am just now said i would try to figure this out. thanks for any help.
thanks
p.s.am writing my posts separate for every issue i have in case u were wondering