10.6.2 "crept in' because it still shows on hardware profile line and it is not supported, there were bugs in the 10.5.7 thru 10.6.2 dealing with processors over-working and heat. Basically we need more details than just the symptoms.
Did you ask and try to get help with the arrangement of and what active adapters you are using to get 3 displays working properly?
With the 5770 you could use a GT120 or another 5770.
Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
http://support.apple.com/kb/HT3382
Readers here have found that if you have two Brand-X Powered adapters, aka active adapters, it also works fine. These do not always require another USB. If designed recently, they can pick up a power pin in the Mini DisplayPort interface.
AMD (makers of the 5770 and 5870) lists the qualified POWERED adapters that meet their most stringent requirements. Look down the page and you will find several qualified Active Mini DisplyPort to various others, such as HDMI. These adapters are available from other outlets, such as Amazon, sometimes for far less than the Apple adapters.
http://support.amd.com/us/eyefinity/Pages/eyefinity-dongles.aspx
The other desktop monitor will plug directly into the DVI port on the card and the 40 inch tv will still connect via HDMI over the second Mini Display port.
That will work only if you use an active Mini DisplayPort to HDMI adapter.
The only (someone) have found to work (2009)
http://www.alibaba.com/product-gs/337561577/miniactive_display_port_tohdmi.html