Hi,
Well, you specified ... "as cheap as possible" ... if it's a new PC ... it's likely to have a PCI-Express slot on the motherboard ... if it's older (possibly second-hand) ... you might need an AGP card. If it's VERY old ... then you will need a PCI card (which are scarce these days)
So the first thing you need to know is, what type of graphics card you need. PCI-Express, AGP or PCI. These are NOT compatible! You cannot put an PCI card in a PCI-Express slot!
Cheapest ... do you need to play games, as in the new games on the market? Then cheapest is not going to be very useful, and you'll land up having to buy a new card soon! Because the cheap cards will not render 3D graphics fast enough. They will be very "jerky"! If the only thing you need the card for is things like Word and Excel, then by all means, you can probably get a card for R150-R250 new. If you need to play some games, I would start with a GeForce 6600GT card. It's a good all rounder. They're usually about R1000-R1300 with 128MB memory on the card. The memory on a card is essential. If you just need to do things like Word/Excel, then you only need 2MB-8MB ... depending on the resolution of your monitor. The memory on a card is used for 2 things.
1) Resolution and Refresh rate. eg. 1600x1200 resolution will need more memory than 1024x768
2) Textures in games. The more memory on the card, the more textures and higher texture quality in games you can have without slowing the action.
Check out my site to get an idea of costs:
AGP:
http://www.bluesting.co.za/category.php?category=12
PCI-Express:
http://www.bluesting.co.za/category.php?category=13
Questions from here?