I'd try & get another stick of the same ram/model as you already have. You can mix & match, but that does not always turn out ok & it's a gamble. Although, most of the time, from what I have seen, it's ok to mix as long as the sticks are some what the same speed. (i.e. ddr 400mhz/w 6-2-2-2 timings)
Same company/brand when refering to matched or dual channel configurations. But I have seen quite a few people that mix & match with no "problems" pursay, but if one stick can only do 225 fsb & one can do 215, your stuck at 215.
Whats this ultra memory you speak of? Brand? Speed?