Re: AMD model numbers; For the AMD HD series up to and including HD5, the HD x870 has been the top end, x670 the mid-ranged, and the x470 the budget range. Within that, xx70 has full spec, xx50 is cut down (to cut down costs by disabling some of the stream processors), and xx30 is rereleased cut down (basically by recycling over tech to get prices even lower). With the HD6 series, the 6870 and 6850 are considered mid-ranged (they're mostly the same as the HD5 equivolents with no new technology, just tweaks) whilst the HD6970 and 6950 feature the new technology and are considered high-end. In terms of cost
ower value, it's pretty much a straight line in terms of scale from their budget options to their flagship models - the more you pay, the better it is.
That information is not totally accurate, because whoever came up with the numbering scheme for AMD couldn't stick to it all the time, which gives reviews a headache for trying to understand exactly where in the scale each model belongs. Still, the end result is the same - the more you pay, the better it is.
Re: "onboard graphics are okay"... I don't agree. Onboard graphics are a fallback option for those who have no graphical needs beyond watching YouTube or BBC iPlayer or their equivolents. For gaming, onboard graphics are never sufficient (Intel HD graphics exempt) for playing games released in the same year. The rule of thumb has always been; if you're playing games on a computer or doing graphical work (image editing, video editing, modelling, animation or rendering), you need a graphics card... and usually, you need a good graphics card. Of course, what you get depends on your budget.
Re: "or you could get [insert console name] to play games"... not all games are on consoles that you can play on PC (strategy games for instance), and not all PC games are on consoles. Particularly, Wii and PC have no games in common. Moreover, consoles are not designed to use keyboard/mouse as a primary input (or rather, games are designed for controllers).
There are many other points I could make about consoles vs PC, but I don't want to spark an off-topic fan-war. In this case, the OP wants a graphics card to play his PC games, and probably doesn't have the budget for an Xbox 360 or a PS3. Focus on that.