The two major video card manufacturers are ATI / AMD and NVIDIA. They both have their own model schemes that you can use to help determine what video card to purchase.
ATI / AMD RADEON HD:
ATI / AMD uses a four digit system. The first number is the series ranging from 2 - 7. The next is the performance band within that segment. Two thru Four (2 - 4) being the lowest performance and nine (9) being the Highest performance. The next two numbers determines performance within each band. Zero (00-50) being lowest performance and (90) the highest performance. The 6450 is the is the lowest performing graphics card in the 6000 series. The 6700 series cards are considered the mid range card in the 6000 series and anything above the 6770 is considered a high performance card. You can find a list of ATI / AMD graphics cards on their website.
Example: The ATI 4770, 5770 and AMD 6770 will perform pretty much the same. The newer model will usually have the ability to accommodate newer technology that was not available in the older version.
Low Performance : Radeon HD 6450, 7450
Mid Performance : Radeon HD 6770, 7770
High Performance : Radeon HD 6850, 7850
Ultra Performance : Radeon HD 6950, 7950
NVIDIA currently uses a three number model structure, but used to use a similar structure to the ATI / AMD cards. Like the ATI / AMD cards the first number is the series. The next number ranges from 1 being the lowest performance in that series to 9 being the highest performance in that series. The next number is usually a 0 or a 5. The 5 usually means that they have added something to the card to make it perform better. The NVIDA GeForce GTX 710 is the lowest performing video card in that series. The GTX 740 is the mid range card and anything above the 750 is considered to be a high performance card. You can find a list of NVIDIA graphics cards on their website.
Example: The NVIDIA 730 and 930 will perform pretty much the same. The newer model will usually have the ability to accommodate newer technology that was not available in the older version.
Low Performance : GeForce GT 710, 730
Mid Performance : GeForce GTX 750, 1030
High Performance : GeForce GTX 1050, 780, 960
Ultra Performance : GeForce GTX 1080, 980, 780