depends on the heat output. ASUS uses a single 120x 38mm rad for its 7990 super card.
but back to your question. you have pretty much already committed so here is what to
expect. 20°-30° drop in loaded application. temperature will vary between ambient, radiator
choice, fan/fan speed and overclock.
your idle will only vary again by ambient and over-voltage. can be anywhere between
10°-30° above ambient (80°F=30°C).
not to impressive, eh? you can always try the H50/60/70 mod to the GPU, looks ghetto,
but can be had pretty cheap.. use some 'sinks for the VRAM/VRM under $100.
or (you don't wanna hear it)
sell TOXIC (100-125), use h2o budget and get into a 7870XT, 7950, 7970. should be more
than enough muscle and won't have to hit it as hard as the 6850 has to.. just a thought..