The i5-2500k beats the PII X4 980 BE in every single benchmark except SYSMark 2007 - Video Creation benchmark. The PII X4 got scored 252, the i5-2500k scored a "measly" 251. However, I tend to ignore synthetic benchmarks since they do not necessarily release real life performance.
The FX-8150 also performs better in the 2nd video encoding pass using the x.264 codec. But it also does a bit better with most of the 3DMax benchmarks. It also does very well with POV-Ray 3D rendering benchmarks and edges out the i5-2500 in the Cinebench test as well. If you compress a lot of files using 7-Zip, then the FX does better there too. The 4 extra cores can come in handy in certain applications. There are a few other benchmarks where the FX-8150 does a bit better than the i5-2500k that I did not mention.
If it must be AMD, at this point your best bet is probably to go with a Phenom II x4 (or Phenom II x6) and overclock it as far as it will go. If you are interested in a cheap as possible system only. There is really not a good AMD processor to build a performance system around today, because they really dropped the ball with Zambezi and it is not competitive for the price.