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God said:
I also put a thermometer up there this morning and checked every hour.  At noon, it was 92ºF.  By 3pm it had gotten up to 108ºF and stayed there for another hour or so.  It had finally dropped below 100ºF by 7pm.  Compare that to 61º-87ºF outside today.
To combat heat, I was planning on venting the room with a fan strong enough to recycle the ~300 cubic feet of air every 3 minutes or so, while pulling in cool air from my bedroom (which I tend to keep around 75ºF, but will probably be cooler as it cools outside).  By the time it averages around 60ºF outside, I'll have the room all set up and start fine tuning my cooling options.
Any advice or suggestions on this proposed setup would be greatly appreciated.

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Couldn't look at your diagram right now because the Growery's image server is being a bitch. Anyways, those kind of ambient temperatures as you were measuring will really make it challenging for you to keep it cool enough underneath a 600 watt HPS. I don't have too much hard data on the temperatures of our 600 watt HPS, but I do know what happened the other day. The thermometer hangs in the middle of the cool-tube about ten inches underneath or so (you're going to want your plants about 12" away from the lamp, if you're closer to 24" you're really just wasting most of your light). With the air conditioner in the next room keeping the thermometer reading 20°C/68°F, I had closed most of the door that leads into the room for a few minutes to accommodate what I was working on, and in minutes, it shot up to 26°C/78°F. From what I remember, ambient in the room when the lights are off and there is no air conditioning is about that, 26°C/78°F, and the temperatures would easily be pushing into the 90's and perhaps beyond.

The recommended exchange of air for the plants to have in general for respiration purposes is replacing twice the volume of air in the room in one minute, so consider that as a general guideline (I don't think it matters too much if you were closer to refreshing it only once a minute). Anyways, I mention this because once we tried to setup the 600 watt HPS in a closet that was much smaller, and the extractor fan had enough cfm's to replace the air in the closet like eight times a minute, and with the ambient from the closet outside was probably around 26°C/78°F or lower, the temperatures inside the closet a couple feet away from the HPS was still closer to 100°F. Even if you succeed in bringing the ambient temperatures in your attic down to 75ºF, I think you're going to have too much heat - maybe not quite enough that growing will be possible, but definitely too much for you to be anywhere near growing them in optimal conditions, since that ambient temperature itself is right where the final temperature should be.

The amount you'll invest on extractor fans attempting to solve this by moving air around is going to be costly, and you aren't going to see too much of a gain for it to be worth it. Your best option is definitely going to be to condition the air instead of moving it. If you are in the States, cheap window A/C units are readily available (in fact, now is about the time places like Wal*Mart will be marking them down as the season is ending). There are even ways to construct some kind of holding box so that it isn't necessary to actually install it through the wall, which I can dig for later if you are interested.

Anyways, that's my thoughts and experiences trying to be able to use our 600 watt HPS. In the end, when you finally get it working for you, where you can actually keep it as close to the air-cooled reflector as you can (we actually experienced light-bleaching because they grew to being 5" away ), you'll love the results.