i recently upgraded ram in my machine and found out (the hard way) that chances are, it won't work. nothing is going to blow up on you or anything, but its best to typically stick with same brand same model sticks. the reason for this is because different sticks run on different latencies and different voltages. you can change voltage settings in your bios usually, but it will change both channels at teh same time, so you most likely wouldn't be able to change the a1 and b1 slot without changing the a2 b2 slot voltages.
i think the best thing to do would be go to tigerdirect or newegg and see what specials they are running. last week i got 4GB (4x1GB) OCZ PC6400 800Mhz DDR2 ram for i think $30 per two sticks, so $60 altogether (with $40 worth of mail-in rebates).