Well this is my impression of the situation:
The current amd top of the range bulldozer bottlesnecks 3x 7xxx series cards. Now i know that only silly people get three cards but i think you'll be pushing the limits with crossfire when the 8xxx series and top of the range nvidia 6xx are released in Q3 & 4.
You could get the secondhand phenom II, i'd recommend a x6 since you can cherry pick the best 4 cores to overclock on since most games won't use all 6. However you will need to get a new cpu in 6 months if you are going for those gpu releases in which case you would have been better off with getting a decent setup now I think whilst the mobo is worth something. Phenom II powering 2 8xxx cards does not sound good to me.
There is no point getting a bulldozer since the 2500k is better value and equals performance of the top bd.
Of course AMD will have released their new enhanced bulldozer then, but whether that's any good is anyone's guess. In fact I'm not even sure it's the same am3+ socket if memory serves and might not be backwards compatible...?
My general advice to people cpu wise (and i think almost everyone on this forum would say the same thing) is:
1. Stay with sandy bridge if you have one - Ivy won't be worth the upgrade.
2. Get an Ivy Bridge if you don't have SB and want gaming at max settings and high performance for general use.
3. Get SB-e if you'll use it for heavy processes like folding/rendering.
There has also been discussion between getting the 3770k vs 3930k for future proof gaming. On the one hand the 3930k does have more spare capacity for the future because of the 2 extra cores so will last longer than IB but will set you back another £250 to build the system today. Also, Intel are expected to release the new Haswell cpus in 12-18 months time which will likely stomp on even SB-e. That doesn't mean that SB-e will be slowing your gaming performance but there is no point upgrading to haswell if you've paid the extra £250 since the gaming industry is only just beginning to make use of 6 threads, let alone the 12 that SB-e has.
Personnally I'll be getting the hyperthreading version of IB, the 3770k. Which will be still pretty cheap (£250 vs £450 for SB-e). You aren't paying for 2 extra cores you probably won't make use of in the medium term but you do get 4 extra half threads which will guarantee it chews through pretty much anything you throw at it - in fact the anandtech benches suggest it will beat SB-e, just, core for core.
Essay over.
M&P