With a 300 Watt PU one of the HD54XX cards is about the only thing you can use i think. You should be able to run 2 monitors on them at the same time no problem. ( though i think it needs to be the same res? not sure on that ).
Any dual monitor card should allow you to run two monitors at different resolutions. I'm running such a setup at work and when you go into windows display properties it lets you individually set the screen resolutions.