x5677 - $75
X58 motherboard - $150 (used)
H105 - $105
Larger power supply - +$25
Total: $355
ASRock B150 - $73 (2x PCIe 16x slots)
i7 6700 - $315
Total: $388
You're right in that you're not going to get triple SLI on a B150 board, but scaling is abysmal past 2 cards, and is almost certainly not worth the costs. Plus, ten times out of ten the i7 will be faster, even than a 5ghz Xeon with triple channel ram. It will draw a fraction of the power, and be running on a new motherboard with a warranty. Better still, there are still 2 more generations of CPUs being released on socket 1151 that may offer upgrades down the road.
That's not to say you might not snag a great deal on an x58 board, or do a lesser overclock using a moderately priced heatsink and throw the economics back in favor of the 1366 Xeon, but there's something to be said for running a new CPU, with new instruction sets, on a new motherboard with a warranty, and be able to do so with a fraction of the power consumption and heat output.
~
Regarding the FX CPU, whether or not it's faster really depends on what you're doing. There are plenty of cases where the 9590 gets beat by an old i3 in games:
There are also cases where the FX CPUs hang with i5's and even approach i7's:
^ While drawing several times the power, of course.
The problem with FX CPUs is that their performance is inconsistent, due to terrible per-thread performance. If you buy an i5, you know you're going to have a solid experience in any game. Passmark scores do not reflect this - things are quite a bit more complicated than a single number can convey - and that's really all I'm trying to say.