Upgrading video card on p7-1421 PC

I'm looking at adding a video card to my pc.  The problem is every card I look at either requires more power or is a dual slot card.  On some forums I've looked in there are a lot of warnings about making sure a dual slot card is compatable not only with the mother board and power supply, but also if it will fit in the case.  Will any of the cards below work with the power supply I have and fit in the case?

I have a p7-1421 PC with a 300W power supply.

The cards I'm looking at are :

MSI  AMD Radeon R9 270 2048MB PCIe 3.0 x16 Dual Video Card

MSI  R7 260OC 1GB PCIe 3.0x16 Video Card