OP, you're not going to be disappointed coming from your C2D to an X6.
And if a 2500k is a bit outside of your price range, then forget about an 8150 as well.
I think there will be two camps--those who advocate for the highest performance, and those who advocate for the "this is still a huge upgrade even though it's not the highest performance." I'm in the latter group (perhaps due to my old e2180 and windows xp...
If you're concerned about the price, get the X6--sounds like you're more concerned about just getting more performance than you have, as opposed to beating nearly everything else that's available. It will cost a decent amount less and net you all the performance you probably need, likely for quite some time.
Keep in mind, though, that if you do go with the X6 and you think you might want to stick with AMD for future upgrades, no doubt piledriver and subsequent progressions probably won't be on the AM3+ socket, so you'd need to overhaul again. If you went intel...I think the next revision will still be on the same socket as current ones (though I'm not 100% sure).
I'm certain that whichever route you go, you'll be blown away by the performance, so in that case I would recommend just getting whichever option is the cheapest for you.