そう after the plain form of a verb indicates you are reporting secondhand information, rather than your own direct observations. 行けるそう means "it is said that it can go." The negative form is -ないそう: 降らないそうです "They say it won't rain."
-そう after the stem (-ます form) of a verb means "looking/sounding as if ___." 行けそう means "it looks as if it can go." It is similar to constructions like 行けるみたい or 行けるよう, but -そう emphasizes evidence of the senses rather than general "seeming." The negative form is -なさそう: できなさそう "It looks as if s/he can't do it."
(Explanation paraphrased from Japanese: The Spoken Language)