How do you tell Graphic card types with words like # series, geforce radeon?

Question

I know nvidia makes geforce, and ATI makes radeon, and SLI is nvidia's "using two cards at once", but how can you tell how good the graphics card is by the Series and name?

Answer 1

Here' my few cents on DX10 cards:
Cheap or budget cards:
ATI -HD 2400 Pro and XT
Geforce 8400GS, 8500GT

Mainstream:
ATI HD 2600 Pro and XT
Geforce 8600GT and GTS

High end or powerful:
ATI HD 2900XT, HD 3850, HD 2870
Geforce 8800GT, GTS, GTX, Ultra