Don't let it feel selfish to Jackie's memory. Rather, think that you are honouring her memory - if she hadn't been such a friend, such a part of your family, you wouldn't want another cat. I've found over the years that you never stop loving the ones that have gone just because there are new ones who need love. And, as you say, there will be a little rescue cat out there who is desperately in need of the love you can give. It's a homage to Jackie that you feel this way.