Know your motherboard

First of all, when you set out to buy a new graphics card, you have to know which type your motherboard likes. There are quite a few slots and sockets out there you should be aware of before visiting the shop. Old computers are often fitted with the since discontinued AGP port, but if you happen to have this, worry not there are still recent cards manufactured. These are usually middle range video cards with a bridge chip included to translate between the native pci-e interface of the main chip hosted on the card and the older AGP interface. You know you have an agp slot when it is brown colored and looks like the one to the left. Conventional color for these slots is brown, but gigabyte, dfi and a few other manufacturers made it a bit fancy, most notably yellow on dfi and purple on gigabyte boards.
PCI-e slot vga cards are much easier to buy as they are almost alone on the market, chances are that if you spend 200 dollars on a vga you’ll get one with this type of connector. Make sure though, asking shop staff or checking the specs on the page of the online shop is the best way to do this. You can see how it looks to the left. Note that all darker yellow slots are pci-e slots on the picture despite of the fact that they are three different sizes. The largest one is the one that provides 16 lanes, your new vga will use this, the rest is for other expansion cards like sound cards or network interface cards. Always make sure to buy graphics cards corresponding to the socket on your motherboard!

There are often power supply connectivity on the video card, this is what powers the unit, because the connector on the motherboard usually is not able to provide enough juice to get it running. Before choosing a card, make sure your power supply has the right type of connectors. Should it lack one, $5 solves the problem so don’t let yourself to be ripped, don’t buy a new supply unless yours is rated under what it states to be the minimum on the box of the VGA. If you have a quality 400w PSU, you’re most probably good to go.

Know your needs and budget

First you need to decide what kind of application do you need your new video card for. In most cases people just want something so that their computer operates and is available for churning some text into that document or spreadsheet. Anything with the right connectors and output channels would fit your needs, make sure the unit you are buying has both. Check your monitor first, and the type of cable it has and pair your new vga with it.
If you are more into gaming, you should think about how much money you have first, because these cards range from 100 bucks to $600-$700. Any of them would be okay for gaming but you need to know how hard the pressure will be on your future card. If you use an old monitor with resolution smaller than 1650 x 1080, you probably don’t want one with more than 256 megabytes of memory because there will not be enough textures (skins to wrap models in, in other words) to fill it with, and you will run out of steam before you run out of memory anyway. So I wouldn’t suggest you to pay extra for something you won’t even notice. SE and LE versions usually are short for Limited Edition in the sense of the word that they are limited in capabilities not the volume of production, should you have the chance, stay away from them as long as you can. It is also highly recommended to check out benchmarks before storming the nearest pc shop.

Chose a retailer you trust, or is to be trusted because this part of your desktop computer is almost always under heavy duty and is likely to break and while however sporadically that may happen, you will still have to be able to get it replaced under warranty.

Once you’ve decided to buy graphics cards and picked up the one you like, installing is not much of an issue, with a steady hand and a cross screwdriver you can do it on your own in 15 minutes. Unplug the power cord and make sure the pc isn’t under any voltage. Open the case, remove any former video cards and insert the new one. Make sure to fasten a screw to the backplate in a way you see on the other cards. This is to make sure your case holds it instead of stressing the socket on the motherboard and you minimize the probability of contact errors as well. Check if there are any power plugs incorporated on the card and connect cables from your power supply before trying to power on the system. Good luck and have fun!