Actually, the E8400 is rated for a TJ Max of 72.3°C. You can read more about the processor here:
http://ark.intel.com/products/33910/...333-MHz-FSB%29
No, the GTX 550 ti and the R5770 are about the same in performance. There is about a +/- 1-2 fps difference in some games. If you want to replace the 5770 with a better card where you will really notice the difference, you need to upgrade to at least a GTX 570 or a R6970, which will both be severely bottle-necked by your CPU. As such, I wouldn't recommend a GPU upgrade.
Either you buy the parts uggorthaholy suggested, and upgrade to a Sandy Bridge CPU (like the i5 2500k), or you wait until new tech comes out (early 2012). In your situation, seeing as how your computer isn't horrible, I'd suggest to wait - but it really is up to you.