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A graphics card is only necessary if you play a lot the most recent video games, do a lot of video editing and maybe some basic 3D rendering. There are other programs and reasons to have one, but if you're one of those people, then you probably don't need my advice.
If you do need a graphics card, you're going to need a lot more help than what the scope of this site will offer due to the vast number of manufacturers, types, naming schemes and configurations possible. However, I'll do my best to at least get you abreast of the terminology and what to look for while you are doing research.
Note : If you are looking to upgrade a computer that is more than 6-10 years old you'll need to find a graphics card that uses the AGP slot. This is an old slot that was part of motherboards and dedicated for the graphics card. If you're upgrading an even older computer you'll need to find a card that will fit into a PCI slot. Current cards are all some flavor of PCI Express and should be backwards compatible to any motherboard that supports PCI Express.
To integrate or not to integrate
Manufacturer
Memory
Ports
Final Notes
About Me
Not every computer has a dedicated graphics card. Many motherboards come with a built in integrated graphics solution. These motherboards use part of the system RAM as memory for displaying graphics. Often times this is not advertised very well when buying a PC, and most notably will trip up the most budget PC's who come with 512MB of RAM and an integrated graphics card that can take up anywhere from 32MB to 128MB of the system ram... you can do the math (I recommend 1GB of RAM minimum for Windows XP).
However, if you have plenty of system RAM then an integrated graphics card might be the solution for you. They are very quiet (in comparison) and take up far less power (saving you money). Also, for everyday tasks an integrated graphics card can handle most things just fine.
One reason an everyday user might want a dedicated graphics card is if they want to run two monitors since generally an integrated graphics card does not provide a dual output.
Bottom Line - This is based on your needs. If your daily tasks include little more than watching videos and word processing an integrated graphics card should suit your needs given enough system RAM. Otherwise if you have needs that require more power and or want to use two monitors you will most likely need a separate graphics card.
SpeedYou have two choices AMD which purchased ATI or Nvidia. Both brands make excellent graphic cards that are comparable in quality and performance. Now the confusing part is that neither AMD or Nvdia sell the graphic cards themselves, they sell the boards to a plethora of third party companies that create variations of the cards while renaming and rebranding them. Each card can still be identified as the base model from AMD and Nvidia but the cards might have been tweaked to be a little faster or have more memory than the original card making comparisons difficult.
Bottom Line - If you arn't a fan of either company then look for the best deals and prices, sometimes you can find one company using rebates or price wars to undercut the other.
This is measured by the core clock and is rated by Mhz, example speeds may rage from 250 MHz for older cards to 700 MHz (and up) for more modern cards.Memory
Bottom Line - More is better but it might begin to cost you for the latest and greatest cards.
Graphics cards have their own memory built into the board. Just like system RAM they have a speed rating in MHz and a quantity in MB (and possibly GB for the biggest cards). There are various types of memory, the most recent being GDDR3 but there are no compatibility issues so this isn't something you have to worry about.Ports
Unfortunately some manufacturers like to take advantage of consumers by touting the amount of memory a card has. However, if you have a lower end graphics card with a slow core clock then it might not even be able to take advantage of all the memory it has. For this reason, look at the core clock first and then the memory.
Bottom line - Memory matters but get a fast card first.
Graphic cards come in any number of variations. They generally will have at least one VGA/DVI output. LCD monitors generally have dvi ports and many come with both DVI and VGA. However, old CRT monitors typically allow for vga input only. You can get a DVI to VGA adapter and vice versa for around $10 from most computer stores. Most graphic cards come with an s-video port that allows you to view your computer from your TV. Some cards also come with an HDMI output as well.Final Notes
Bottom Line - If you have an old monitor make sure the card has a VGA output or you'll need an adapter. For most people though a single DVI output will be all they'll ever need.
Graphic cards are tricky because both companies use naming conventions to make it difficult to know which cards are the high end and low end especially when you take into account the third party companies rebranding the cards as their own.
Also, graphics cards are being released constantly at a premium which will drive down the price of the last series of cards. For this reason you should never buy the latest graphics card because its price to performance ratio will be far lower than the last generation of cards.
I would recommend reading Research Tips before making any purchases and do your background research very carefully before buying a graphics card because they are one of the more expensive components of a computer setup.
Who am I to be telling you what to buy with your hard earned money? Check out the about page to decide for yourself.
Bottom Line - Check it out. I even talk about my own computer buying experience!