There are too many numbers to avoid, let's offer the numbers to look at.
ATI: older 9600 Pro (or some suffix denoting "turbo), x1600 (probably not as good as the x800), x800 (any version, also x850 I think), x1900/1950.
NVidia: older 6600 (rather low end, like Ati 9600), 6800 (all versions), 7600 (GS okay, GT almost as good as 7800!), 7800 (all versions), 7900/7950.
Top of the line is Ge 8800 (any version) but that's PCIe only and too freakin' expensive for the normal person.
Avoid all other numbers...
You come across an "x1400" -- stay back!
You see a Ge7300 -- run away!
They make a "good" version.... One or two versions will come out with the release of a new card. Then they fill in the other numbers with cheap, stripped down, POS, cards, to fill the budget market niche. These cards are not even a fraction of the full card, sometimes. Most can't do 3D that well due to lack of VRAM on the card itself, slower clock speeds, and/or low power in general.
Example: The premier cards for the Ge7000 series are the 7800 and the 7600. These were the first. After this they started making 7300s (total crap) and other things, that use the new chip architecture, but are completely inferior (hey, they're budget! Not built to be good!). Then there's the 7900 and 7950, both powerhouse cards, but they came after the initial release.
So, for example, there's the x1600 with ATI, but they also put out an x1300, which is totally inferior to the x800 (in general). Higher number doesn't mean better card. Ge 6800 is probably far better than Ge7300.
Sorry if that made things more confusing.