Theres almost NO reports of those running up to 100c. They all aren't going to run the same, but something doesn't seem right ? If your considering changing the heat sink, I would take the card out and physically inspect the mounting of the stock hs.
Can you take a gpu-z screenshot while furmark is running ?
Go to sensors tab, in the right hand corner is a icon of a camera, click
it asks you to u/l to free pic hosting site and gives you the url, post here for us to see.
IMG]http://gpuz.techpowerup.com/10/05/19/cz9.png[/IMG] just copy and paste
Undervolt the card, i got that horrible egg shaped one and it ran very hot, close to 80c and i undervolted it all the way to 1000 (mV) in MSI afterburner, stable and running at 61c full load with 55% fan speed.
Adjust you fan speed a bit to about 60-70% and you will run it about 65c max under load and idle about 35-40c. You can adjust your fan speed with Riva Tuner, ATI Catylist Controller or the MSI Afterburner program.
Yeah when running FurMark you gotta set the fan to really high, like 80 or 100%. It will be loud and annoying, but FurMark heats the GPU up like nothing else. Gaming wise, what are your temps like?
Anyway, which cooler to get if you really want aftermarket will depend on your space requirements. If it needs to fit into the standard 2-PCIe slot space, then your options are limited to basically Zalman VF1000 (or GV1000) or Scythe Setsugen. I have Scythe Setsugens on my 5850s and gaming temps are at about 60C, FurMark around 70C and they're super duper quiet.
I understand this is solved now, but I am on my second XFX 5770. The first one hit 97.5C according to MSI After burner with fan speeds matching temps.
My replacement XFX 5770 just hit 75C according to MSI after burner after two hours of Skyrim on high settings. The old 5770 was in my old oven of a case, the replacement is in my Lancool PC-K62 case.
I don't have a screen shot of my old card hitting 97.5C but I have MSI after burner logs which of course can be manually changed but why bother?
Also, the 6770 is the same card just re-branded so to speak. So in theory the 6770 coolers should work on the 5770. There are more than the Arctic cooler listed above that will work on the 5770 but I can't read German.