Yes, get the XT for Crossfire...As for overclocking, I use ATI Tray Tools which will actually find the highest overclock for you. You basically increase the MHz until it shows artifacts (it will tell you). One you reach that highest possile speed, decrease it to 10 MHz below that and you should be safe. Other people use Rivatuner which I hear is good too and probably works about the same. Download them from www.guru3d.com
Your speeds should reach or even go over the XTX speeds. I've read that even if you don't increase it all that much, it's still going to increase performance alot. Something about the difference in MHz being much bigger than previous cards. Probably something to do with the new architecture.
Let us know what kind of speeds you get. Oh yeah, as for how hot should you let it go...Most people don't like their cards running hotter than 70. I keep mine overclocked and maxed out and it doesn't reach higher than 65 with the HIS cooler. Search the forum for more info. on that. I think there's a sticky somewhere about all of this.