I'll try to explain it better, but be aware English isn't my native so i usually find myseld in hard spots :P
Every year or so (sometimes even 6 months) ATI and NVidia release a new generation of VGA and thats wehen we go from 6000 series to 7000, 8000, 9000, 200, 400 (being 400 the latest for Nvidia)
Ati has series 1000 2000 3000 4000 and 5000 (and of course there was a series 9000 before those which is INFERIOR)
Lower tiers are not intended for serious gaming, you'll be able to run every game but not mostly not in medium settings (this includes x300 to x400 being x the generation) then comes Mainstream being somewhat x500 to x600 and after that you got serious gaming card which are intended for gamers who actually want high resolution and maxed settings on most games).
You'll noticed i didn't say anything bout series x100/x200 becouse those are generally (not sure if always) integrated cards, which of course come within the motherboard itself. those are just for real low low low settings on mostly everything except oldies.
Now any low tier card from this year WILL let you play the game but i can't reccommend those if you intend gaming and not only for sc2.
There's a lot you can google about cards, try looking at tomshardware vga charts and google a bit to find which card is good for you.
PD: by PCI they actually mean PCI which differs from PCI-E or PCI EXPRESS being this the standard in the industry now.