I guess first thing is to check if you have an AGP slot on mobo (usually top one and brown) - which would give you options for that type of card. Then check if you have PCI or PCI express slots (white ones) in case you use a PCI card.
Then there is op system compatibility - which should be mentioned with a card's specs.
Need also to know what your PSU rating is as often necessary to upgrade that to higher power unit.
Probably also worth checking on card output - whether VGA connection for cable or a DVI - and whether your monitor will suit. etc. There's more but this is a start.
