If you used a real program that actually is made to listen to music and not **** around with your computer (such as foobar), there's a feature called Auto Tag that would force you to press two buttons, and bang, everything set.
So my recommendation is:
a) close iTunes
b) go to www.foobar2000.com
c) download, install, run, add files
d) highlight all files with the same artist (even if they have different names like DragonForce, Dragon Force, Dragon_Force etc)
e) replace the artist name with, say, "Dragonforce"
f) close foobar
g) start the piece of sh- I mean iTunes
h) sync
i) enjoy
edit: realized you bought the low quality versions from iTunes store, there's nothing you can do then. Sorry mate!