I've heard that there are some difficulties using multiple AMD cards. Before I go buying a second card, I'd like to know if I can use them as one big desktop, with different resolution monitors. I don't need 3D across the monitors.
Why would you need a second card if you don't care about 3D performance? Any HD5xxx should be able to drive 3 monitors without trouble.
- are you going for windows gaming with crossfire? Simply don't use the second card on linux.
- are you going for a 6-monitor setup? Xinerama should do what you need (with the usual problems: slow speed on windows that span multiple cards, bugs when using a compositor like compiz). The cleaner solution is to replace the HD5450 with a single card that already has enough outputs.