Humans hate pows and programmers hate humans - let's connect them!
Python 2, 204 190 bytes
This is my first ever codegolf.
s=raw_input()
p=' KMG'
if s[-1]&lt;='9':
 e=0;r=int(s)
 while r&gt;=1024:r/=1024.0;e+=1
else:
 e=p.index(s[-1]);r=eval(s[:-1])
 while e&gt;0:r*=1024;e-=1
print(`r`if r%1!=0 else`int(r)`)+p[e].strip()