Upgrading from a GTX 645 to a R9 270X

Oh and it is the r9 270x 4GB version not the 2GB. 

The R9 270X does use more power than any card Dell has tested on this system.  The power requirements between a GTX 645 and a R9 270X will be pretty significant, but you should be okay with the stock power supply.  You will have to use both your 6-pin PCIe power connectors for the card.  Make sure you uninstall the Nvidia drivers and other Nivdia software before removing the GTX 645.  I would look at getting the Sapphire card HERE if choosing one.  It comes with BF4 and it's not oversize like the Gigabyte card.  Note that you may have to purchase a longer SATA cable for either your hard drive or optical drive as the stock ones my not fit around the large video card because they'll be too short.  

I was also looking at that same card but from other threads like this one: http://en.community.dell.com/support-forums/desktop/f/3514/t/19528647.aspx

they talk about a watermark of some sort on the screen since all I was planning to do was remove the old drivers then put the new graphics card in and install the drivers. Is it really as complicated as it seems?

It's not that difficult.  However, things go much smoother if you have Windows 7 when you don't have to be concerned about secure boot.  When you update the bios on a computer or a video card you have the same small risk such as a power loss during the process which in turn can brick your boards.  I have a UEFI bios with an older AMD card and it starts right up, but I run under Windows 7.  You can contact Sapphire to see if the particular card you are planning to buy has a bios compatible with secure boot in Windows 8 since flashing a video card bios can be more complicated if they even offer and update than a computer bios.  Most of the time just disabling secure boot if necessary resolves the issue without having to do anything else.