You could fit two plants in 1 gallon fabric pots. Put something under them to catch the water, like a plastic saucer.
You should drill out the drive bays. Leave the center bar in for support.. You can get a plate for the power supply and have a fan there. it wouldn't be too practical with two fabric pots on the bottom blocking it's flow, but it probably wouldn't hurt either.
get some fan filters: cheaper plastic black ones. replace the inside foam filter with carbon filter cloth.
get a fan controller. 25$ powers 4 fans.
what i wish you could fit in there is
but not all pc's can fit them and cool them properly.. They require a 120mm fan on top of it to cool it.
Edit: t5's would be sweet as helll. they make some nice 2ft strips for the sides. You could even get some 1ft cabinets t5's for the top and put aquarium 1ft flouro's in it. hagen makes some pretty decent daylights in their glo collection, however, the 1ft wont be High Output... So you could essentially have sick T5HO side lights and 1ft T5 on the top.
You can get some high kelvin(blue dominant) lights at the petstore for mixing in with the 2700 side lights.. like these:
