SUGEI is a kind of slang way to say SUGOI. NA in this case is the same as NE, if you know what that is. NA is stronger and more "masculine" than NE, but you could also easily say,
SUGOI NA!
You can use the SUGOI/SUGEI paradigm for many adjectives ending in AI or OI in Japanese. For example,
Often when you speak this way, you kind of lengthen the consonant immediately preceding the vowel change, so the above examples turn into the following: