1) A Phenom II x6 is a terrible upgrade over your current processor if your primary purpose is gaming. You'll see little to no performance increase.
2) Intel CPUs are nice, but Phenom IIs are good, capable processors. I wouldn't upgrade CPU unless you feel a specific need.
3) A second 6870 would be useless... you won't even use much of your current GPU's capacity at your resolution. A single 6870 will be fully used at 1920x1080, but it'll still do well. Plus Crossfire sometimes causes microstuttering and other problems.
4) Biggest and best upgrade you've considered so far... given upgrades #1,2,3 & 5 will provide no performance benefit for gaming (only 2 is even worth considering, and it's pretty minor).
5) Useless unless video encoding is a high priority for you.
You could also get an SSD, as others have mentioned. If you're happy with your current monitor, that's your best choice. It'll make casual CPU use seem fast and awesome for anything on the SSD (by lowering program load times). But if you get an SSD - get one large enough to hold all the programs you use regularly. A Crucial m4 128gb is under $150 right now. I'd probably get the bigger & better monitor over that though, to be honest,, which will then allow you to use your current one as a secondary (non-gaming) monitor.
Seriously, don't even consider 1,3, or 5. And #2 is a pretty small upgrade. Your CPU is fine.
Edited by MisterFred - 4/25/12 at 10:50pm