The hx520 will be a 'slight' upgrade from the FSP you have currently.
The most major upgrade you'll see will be the modular cables.
That being said I just ordered the 520 from buy.com today so I'd be the teapot calling the kettle black if I told you not to do it.
However I have to wonder if you really 'need' it. The FSP being a quality PSU, is probably enough for just about anything you can throw at it.
You could hang on to it, until you need to upgrade. Or you could jump on the HX520 now, before the deal runs out. Keep in mind there's almost always another deal around the corner. You should buy to suit your needs, not need to buy to suit. You'll find yourself in 'buyer's remorse' more often when you purchase something you don't really need before you are going to use it.
All that being said, if you have another computer you can throw the FSP into, or think it could make up some of the price of the 520 by reselling it, I'd go ahead and do it.
silentpcreview.com currently recommends it as the "quietest PSU over 400watts that has a fan"
you can read that review here:
http://www.silentpcreview.com/article692-page1.html
here's an excerpt from the review if you are a little lazy feeling at the moment:
"
CONCLUSIONS
The new Corsair PSUs are not only aimed at the power-demanding PC gamer, but also the growing class of quiet PC enthusiasts who also want to be able to run some seriously powerful gear. The HX620W and HX520W deliver clean, ultra-stable power under a wide range of demanding conditions. They also happen to be about the quietest fan-cooled PSUs we've tested to date, matching the best of the Seasonic S12s. They are also energy efficient, matching and even besting many of the 80 Plus certified models we've tested. The modular sleeved cables ease the challenges of keeping the system tidy and aerodynamic for optimal airflow, which is especially important if you wish to maximize cooling with the slowest of fans for the lowest noise. To top it all off, Corsair offers a five year warranty, which is the longest for any computer power supply we know of.
Given the manufacturing source, the most obvious comparatives are the Seasonic M12-500 and M12-600 models, which have the same basic circuitry and modular cable system, and probably the same noise-conscious fan control circuitry. However, the M12s have that extra 60mm fan. Although the 60mm fan is there for good reason ? to clear a hot zone that seems endemic to all 120mm fan PSUs ? once they turn on, admittedly at a high power load, they do not turn off. The small degree of noise the 60mm fan adds to the M12's signature at higher loads is enough for us to favor the S12 or S12 Energy Plus series in the Seasonic line... but they lack the modular detachable cables of the M12. Which leads us back to the Corsairs: They seem just about perfect for a quiet-loving power PC user who wants to have his cake but not hear it as well.
About the only thing SPCR could ask for are a couple more lower power models for those of us who seek to make a high efficiency, quiet, yet highly capable computer. 300W would be plenty for such a PC in these days of improving CPU efficiency. Perhaps if enough users pester Corsair (and Seasonic?)...
The final words: Great job, Corsair. "
