My computer's in the basement, and because the house has central air it sometimes gets as cool as 20C - in the room (that's 69-70F). Inside the case it stays about 23C, and the CPU runs 25-28C depending if it's working hard or not, and - yes - it's aircooled, with two intake fans (down low near the drives [RAID 0]) and three exaust fans (not counting the PS fan); two exaust out back and one in the top (I cut another vent for a 'blow hole', but fan stays internal). I don't know what the temp of the video card is, but whatever heat it generates is removed rapidly due to the 'flow-thru' ventilation.
Haven't tried to wind up the processor much though, it just putters around at the 'factory set' 3.06 Gz - but heat isn't really an issue till it gets over 60C by the processor (which seems unlikely, especially since 4 of the fans run at low speed till case temp exceeds 40C).
However, for all that the air cooling does, it WAS about $60 worth of fans (plus time with a drill making a blowhole). Because the fans are thermostatic, and usually run on 'low', it's not very noisy either.
Enough heatsinks and it seems that you'd actually BLOCK some of the airflow (plus the weight could be a bad thing).
Tom