Your main thing will be heat.. I've got 70 watts of cfl in a mini tower and it hits 85F.. (67 in room).. 3 120mm fans, 2 80mm fans.. should note that it is stealth though, ZERO light leaks allowed so the air intake is tricky..
If you have the ballast remote placed I'd bet you can manage it.. i've seen a REALLY good pc grow with a 70 watt hps scrog before.. cant seem to find it right now, but it is possible.
If i find it ill link it for you..