First sli setup help

Hi, i have a question on SLI. I've just baught a second evga gtx 275 in order to sli my system. I already install the latest drivers and enable sli mode in
control panel. I was playing a few games to see if my fps had improved but i dont see any difference for example Cod4 i get 91 fps with sli which with one card i was getting 91 fps in the game. What do you have to do to see any improvement? Im thinking i baught the card for nothing, i mainly baught it because i have a 24 inch monitor for more fps in games.
my pc specs:

cpu - E8400 @ 3.6ghz
mobo - evga 780i ftw
psu - antec 1000w
gpu- evga gtx 275 x2
ram - 2x2b 1066 mhz corsair
cpu cooler - zalman 9500
vista 64 bit.

yes ive done that. but looking at my set up will i get better fps with 2 gtx 275 like if i play cod4?

@ 91FPS with 1 card you wasted your money on the 2nd.