HD3870X2 in crysis lower fps then 3870 ?

i just put in the hd3870x2 in my pc pull out the hd3870 512 mb ddr3 not 4...on single player #11 assualt on crysis. your in the jet looking at a guy talking in a nano suit..i used to get 22-23 fps...im was using the newest catalyst for both.. i put in the hd3870x2 1 gig.. and now im getting 12-13 fps..pretty much lower all around... in c0d4 i pick up from average 60 to 85... but thats not the problem games at 60+ play fine.. i have my setting on crysis very high and a couple at low "dont like the effects" res 1440x900.well... what the best 1 or 2 cards i can get for the $..should i have put in "2" 3870 ddr4. there only 190 each on newegg...or try nvidia. i have e8400 at 3.83 oc.. asus p5k..4 gig mem...620w.. vista 32bit..160 gig hd..ty head