Video cards are fairly easy to install, however if you mess up I take no responsibility for that. Unscrew the screw or screws, push a little tab on the motherboard to get the old one out, push the new one in, screw up the screw or screws. If you have to use a lot of force, something isn't working right, slow down and check what you're doing makes sense.
You do have to check the specifications, some people will be delighted to sell you a weak video card for twice the price that someone else (or maybe even they themselves) will charge you for a stronger one, it's not nice, but you can't just buy on price because of it.
There seems to be a system with video cards that the first part of the number is the year (because of this, you can't assume that a card whose number starts with "62*" is better than a card whose number starts with "59*", it most probably won't be), the second part is the main description of the strength, and the third part is the minor description of the strength. Then, for Nvidia, you have "ti" or not on the end of the name, and other variables too. All in all, you do have to learn what the numbers/names mean, it's not fun, but for whatever reason marketing departments seem to like to confuse things.
These websites are not perfect, but they are some help:
http://www.tomshardware.com/reviews/...ew,3107-7.html
http://www.gpureview.com/database.php