I have a RAM question......

So I thought I would be hot shit and go out and max out my RAM.
My PC is quick already but it was one of those things on the wish list some day due.....

So I currently run 2 x 256 mb sdr 133 and a single 128 mb all three are double sides and $$$$...so I thought I would be all that ....and I went out and got one more 256 the same as the other ones but.....now when I go to boot up it adds up the RAM and then sits........and that's it......

I have removed all three of the cards and tested them all to make sure they work separately but together ....nuthing....

Is there a setting I need to adjust or something....Help....
Ronica