I am planning on buying a new video card, but I am completely messed up with this PCI Express x16 thing. Because in the store there are different kinds of these outputs. For example PCI Express 2.0 x16 or PCI Express 2.1 x16 or PCI Express 3.0 x16. My friend said that it doesn't really matter, but then what is the point of making the outputs different. So, my question is, do the video cards need to match those three or not? And also, what other things I should consider before buying a video card?
The PCIE bus is the same whether it's 1.x or 3.0. Video Cards are backwards compatible. As far as output go, it all depends on what your monitor has. Most video cards will come with a DVI to VGA adapter if all your monitor has is VGA. Otherwise use HDMI or Display Port.
As far as which model of Video Card to get greatly depends on your budget, what's available at your store, what your PC specs are and what kind of PSU you have.
PCI Express is basically an data and power interface between video card and motherboard. those numbers (1.0, 2.0, 2.1, and 3.0) are mostly concerned about bandwidth of this interface. Basically, 3.0 provides twice the bandwidth of equivalent 2.0. For now id doesn't really matter, because 2.0 provides sufficient bandwidth. They are compatible with each other, (the only exception may be 2.1 card for 1.0 slot in some cases). So, 3.0 card will work in 2.0/2.1 slot, and 2.1/2.0 card will work in 3.0 slot.
PSU: open your case and check powersupply for labels.
Read the details on the sticker on the side of the power supply and post them so we can see how many watts on your 12 volt rail and what brand it is. Very important information for us to make recommendations.