Should I relocate the HVAC unit in the attic

1. Check your unit's warranty. The heat exchangers on some units are warranteed for 20 years or longer. You may be able to get at least some money from the manufacturer, or a credit toward a new unit of the same brand. They may also cover all or part of the installation labor.

2. Find out whether there are any federal or state tax incentives that could help. A federal tax credit has been available in the last few years for improving your home's energy efficiency, and replacing an older heating unit with a higher efficiency model was one way you could qualify. I don't know if that's still available in 2012, but worth looking.

3. Your local utility may also offer some sort of incentive for installing a high efficiency unit (not to mention the lower monthly bills).

4. Some utilities have programs like this one, where you can save money on your electric bill by letting the utility reduce your energy usage during peak periods. (In simple language, that means they'll turn off your air conditioning for short periods on very hot days to help avoid brownouts.) This probably won't affect your choice of a new unit, but it's something that you might want to consider at the same time.

First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.

I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.

Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.

Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.

This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.

One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.