currently i don't do much cardio (walking to class etc), but i do lift weights 3 to 4 times a week. i have put on a decent amount of weight since the summer, but i want at least 20 more pounds. i feel if i do cardio it will take away from this weight gain. i'm already eating so much (around 3500 to 4000 cals a day) that if i did start cardio, i think i'd start to lose weight.
if you want to gain weight i'd do compound lifts (deadlifts, benchpress, squats).
how many calories are you eating a day? if you're not seeing any weight gains, i'd add 250-500 calories a day until you start seeing any improvement.