So overall higher is better then? Thanks.
Sometimes it also depends on when it is release. Something that are release later might adopt newer better tech like Radeon HD 4770 would have 40nm GPU instead of 55nm GPU so many would get a Radeon HD 4770 than getting Radeon HD 4830. Radeon HD 4770's performance isn't very different from Radeon HD 4830 and it offers better bang for buck than Radeon HD 4830. It also depends on how long you can wait until the video card that you wanted to buy are going to be release too.
Sometimes being newer generation and having higher numeric value doesn't always mean that it is faster than older video card with lower numeric value. For example: Radeon HD 4550 isn't really faster than Radeon HD 3870 X2, despite that HD 4550 is a newer generation video card and has higher numeric value "4550" and this is because you are basically comparing different cards of different generation and of different category level. Thus, you got to compare them correctly based on their category level like entry level cards of newer generation must be compare with older entry level cards like HD 4550 VS HD 3550 or mainstream comparison like HD 4650 VS HD 3650 then you will see that newer cards have better performance than the older cards of the same category level.
However, it is also true that according to the comparison between the GPUs of the same family generation, the higher the number, the more performance it has but that does not always mean that it will have the latest better tech or offers the best bang for buck so I don't think that looking at the numbers only and judging that it is better in terms of value/better in price per performance is a smart thing to do.