As has already been tossed out there, Eden studios will work WONDERS *has sucessfully run a few to date*.  However, for those who want another system, BESM (Big Eyes, Small Mouth) would do as well if you toned it down.  Fung-suei comes to mind for the sheer of the wall stunts people like Marcone have pulled (like hitting a rope with a knife while being tied to said rope, hanging upside down and spinning in a circle).  I'd toss out D20 modern due to levels.  For DnD it's great.  For something like this, were you're merits lie in how fast you can think and not how hard to can hit, it leaves something to be desired.  And for those White wolf fans out there (this is m too people)  Sorry, but WOD just won't do.  You can make it work, sure, but honestly, Harry as a Mage just doesn't really jive.  It's almost too supernatural for this setting.  BUt there you have it, a quick synapsis from my opinon