Can i mix 2 different company's ddr2 ram? ?

hello. i just came from black friday and am dead tired so i apologize, i wont be able to respond to answers right away. so i bought 4gb (2gb each chip) of ddr2 ram for my laptop. my laptop came with 2gb ddr2 ram and in doing research, it can handle up to 3gb of ram. so here is my question. if i take out 1x 1gb ddr2 chip (one possible company) and replace it with 1x 2gb ddr2 (possibly another company's) so 1gb plus 2gb equal 3gb, will that work? that question was to see if mixing different companys ram is ok because i dont know what the ram is made of. i hope that was clear. and in the answer to that last question is no, i have one more. if i am not able to mix companys ram, can i take out all my current ram and in stall the 2x 2gb ddr2 ram? i think that installing a total of 4gb of ram, 3gb may only be used. is this guess right?, having 4gb ram wont like overload my computer or anything? thanks a whole lot. happy thanksgiving and safe shopping