Well for starters, my 4200+ X2, A8N-32 SLI Deluxe system with my X1900 would heat your 73 degree room about 5 degrees hotter easily. I almost always get hot up here in my loft when that system is on and I'm just glad there's a window and that I'm a guy (shirt comes off). As far as not using thermal compound, I wouldn't recommend it, but with a CPU that doesn't put out a ton of heat it is possible. As long as heatsink is close to the source and the heat concentrates onto it where it can be quickly wisked off by fresh air, it IS possible to go without a heat transfer putty. Today's CPU's run pretty hot, especially dual cores. That said, I don't know what the temp range is for Core 2 Duo's, since I don't have one, don't care, and won't be buying one (I have 2 socket 939 computers, I'm waiting for quad core now). I do know that on my X2 the temp ceiling is about 70 degrees celsius and that sounds about right for the Core 2. AS5 is less viscous than previous versions of Arctic Silver so it stays where you put it instead of slowly seeping out the sides with vibration. It also contains finer silver, and I think perhaps more of it. (someone can clear me on this) Silver is a prime conductor of heat. However, other thermal compounds still do a fine job, such as the stuff you scraped off your heatsink. Today's compounds for the most part are all fine unless you're OCing or your stock fan happens to be crap. On more generic brand I'd say go ahead and throw on a compound you can be sure of, but if a name brand heatsink like Zalman or one of AMD's heatsinks comes with thermal paste I'd say go ahead and use it. Making sure you're using AS5 like some people probably do is more of a peace of mind thing, and wanting to be sure you've got the best stuff.
73 sounds a bit warm to me, but maybe Conroes run hotter? How many and what size case fans do you have? If the air is being circulated inside of your case by that CPU fan instead of directed out; your internal case temp might be too high. A big 90 or 92mm fan or two can do wonders to keeping your mobo and CPU temps down.