Just some pointers to help you:
The only reason to upgrade your lights to MH is to keep SPS. It's possible to do in a Nano Cube but you run into possible heat problems (water to hot, mainly in summer months) and you have to keep up on the water quality a lot more with SPS.
Mushrooms are softies and you right now have more than enought light for them.
I keep everything from zoa's, blasto's, Candycane, Acans, Torches, Mushrooms and a pagoda cup in mine. With the stock lighting you can keep ALL softies and 90% of the LPS corals out there (The only one that might need more light are micromussa's but even then I think you could still keep them.)
My point is don't mess with the lighting unless you are 100% sure you want SPS.
I added a MaxiJet 1200 in place of the stock pump. I heard stories of the stock pump running hot (and raising your tank temp) and being an all around piece of junk. Love the 1200, it moves plenty of water.
Toss the sponges and get filter floss for the back chambers. I don't think you can ever get all the crud out of the sponges and that leads to nitrate problems down the road. For $7 I got a bag of filter floss @ Dr. Foster & Smith and it should last a year. Just throw the used stuff (it will turn white to brown) when you do weekly water changes.
The hydor flow is a good idea. I actually bought one but I think I messed it up when I was playing with it so I have never installed it (yet, it's on my to do list).
I have a photo album of my cube here
http://s61.photobucket.com/albums/h73/colinadam/
It has newer and older pictures, the newer ones are toward the beginning.
If you run into something and have a specific question feel free to shoot me a PM, I'd me more than willing to give you my advice.
Keep us updated
