With most things related to graphics cards, the higher the number the better, and that usually includes the price.
Tom's Hardware Guide has comparison charts so you can compare the real life performance of different cards.
TechPowerup has a GPU chart which gives all the basic info you need to compare different chips.
(You can't tell how good a card is by reading the manufacturers description; they make even the crappy ones sound brilliant)
Before taking the plunge you need to know some basic stuff about the pc you're going to put it in.
Will it take a graphics card? (some compact cases don't have room, and some Dells simply cannot be upgraded)
Has the motherboard got an AGP or a PCI-express slot for the card?
Will your power supply run the new card?