So you already have an HD5450 graphics card and want to know if you can replace it, or do you want to add an additional card and run them in crossfire? I'm kinda confused on your question.
If replacing, you usually can buy a higher end card. Things to keep in mind are your card's slot configuration (AGP vs. PCI-E) and that you have a power supply capable of running whatever new card you choose.
If you want to run multiple cards, I don't think the HD 5450 is crossfire capable and so not possible even if your motherboard has an extra open graphics card slot.
Not sure if I helped here or not. If you still need help, please list what your system specs are, and exactly what you are hoping to do.