Some Samsung is a bit better at similar prices, but Crucial going to be no problems - especially isnce your current stick is crucial aswell, probably more likely to have problems mixing crucial and kingston/samsung than using 2 identical sticks of any of those brands. Saying that, unlikely to have problems when mixing those brands anyway. Personally I'd stick with crucial for aforementioned reason plus cant be sure what you get with Kingston, and even if you get some excellent chips on the kingston stick, well as i understand it the crucial stick would have to be able to match it in your system anyway - so no gain from a "good" kingston stick, and so can only lose from the possibility that the kingston stick be sub-crucial quality.
I have 3 crucial sticks of 256mb PC2100, most recent one has gone bad but got email they received my RMA so I'm expecting a replacement anytime now Had no problems with the RMA at all (presuming they actually do send replacement ) though it probably helps I bought it direct from crucial website.