I would get a 2500K CPU, a suitable Z68 motherboard (with slots for 2 cards, and with slots to accommodate your soundcard a well, so the gpus don't block it), 8 or 16GB of RAM (RAM is cheap) DDR3 1600Mhz or more, I would probably get a 7970 GPU, or a 7950 in crossfire, or a GTX570 or 580 given my money. I would also consider replacing your PSU dependant on its age, as the capacitors age you generally lose output capacity on it.
IMO you could overclock your current CPU more than it already is, maybe invest in some aftermarket cooling, and get a pair of 7950/70, 6970 or GTX570/580s. You ideally want the 3GB 580's and the 2.5GB 570s, and other than that, the 2GB cards. There is not a lot of difference in gaming between Bloomfield, Sandy Bridge and Sandy Bridge E chips, and you already have a quad core CPU. Your 6GB RAM is more than enough for gaming at the moment, as long as you have enough VRAM. I would personally swap the PSU to something new around 1000 watts, if you want something high end with leg room, an AX1200 would be perfect, and overkill for any system (save for 4 GPU dual CPU monsters).
However, you said high end!
The highest of the high end would be the new Sandy Bridge E chips, the 3930K looks a good buy, tag on 16GB RAM (quad channel 4GB/channel) and a suitable motherboard to that, and the above GPU recommendations. A lot of people notice microstutter, on my 5850 crossfire rig I only have really played TF2 and the framerate is so obscene I haven't ever seen any, so I can't say how bad it is but most people say SLI is better than crossfire for the reason of less microstutter, but YMMV.
My best advice would be to say upgrade the GPUs and keep your current system, you won't notice any difference for gaming, but do what you want!
And whilst you are at it, that X-fi card you have is poor, it doesn't have any hardware accelerated features/ DSP etc, it offloads everything to the CPU. You could do a lot worse than use an X-Fi titanium, or maybe some other card, if you are concerned about the absolute best performance.
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