Intel i5-2500K $227
http://www.scorptec.com.au/computer/39853-bx80623i52500...
Asus P8Z68-V $200
http://www.scorptec.com.au/computer/41911-p8z68-v
Corsair Vengeance LP 8GB (2x4GB) DDR3 1600MHz CAS9 1.5V CML8GX3M2A1600C9 $80
http://www.scorptec.com.au/computer/41589-cml8gx3m2a160...
HIS Radeon HD6950 IceQ X 2GB HIS-6950-2GBD5-IQ $295
http://www.scorptec.com.au/computer/41737-his-6950-2gbd...
26cm according to the HIS website.
Antec Truepower New TPN-650 80Plus Bronze Modular $130
http://www.scorptec.com.au/computer/31326-tp-650
Or
Corsair AX-650 80Plus Gold Modular $190
http://www.scorptec.com.au/computer/42095-cmpsu-650ax
6950s aren't that power hungry so it is perfectly possible to run two of them on a good 650W PSU
However, if you want a 750W unit then I would suggest either of these:
Toughpower XT 775W 80Plus Silver Modular $165
http://www.scorptec.com.au/computer/37159-ttpx775
Or
Antec High Current Pro 750W 80Plus Gold Modular $195
http://www.scorptec.com.au/computer/41497-hcp-750
Samsung Spinpoint F3 500GB HD502HJ $80
http://www.scorptec.com.au/computer/35078-hd502hj
HDDs are quite expensive atm due to the flooding in Thailand, so personally I would try to get by with as little storage as possible in the hopes that it will last long enough for prices to begin to fall again. If you need as much storage as possible immediately then you will have just accept the higher than normal prices.
Any SATA DVD writer, unless you want Blu-ray. ~$30/90
Total: $1047-1107
Then get a high end 24-27" LED IPS monitor. Unfortunately I don't know the monitor market that well so I can't make a recommendation.
You might also be able to afford an SSD for faster booting times. This depends on how much you spend on a monitor and how much you want to use all your $2000 budget.
The Apollo is going to restrict your graphics card choices, this is because, according to the NZXT website, it is "8800GTX compatible" (which, if like other Nvidia cards, is 10.5"/26.7cm long), whether this means it can take longer cards than that I don't know, but I would think 10.5" would be the limit and a sensible limit to stick to unless you know that it can take longer cards, which I don't.
I also think that it wouldn't be the best choice for a crossfire system, as the airflow doesn't look that effective to me.
So I think that you should look around for other cases which are less restrictive and if you can find one that you would want to replace the Apollo with then I you should do so, but if you can't then you will have to be prepared to accept the limitations of the Apollo.
Heat and noise are the major downsides of crossfire, occasionally newly released games might have driver issues for a short time before the driver can be updated. Personally I think that unless you are running very large resolutions (3 displays) then it is not necessary to have crossfire except as an upgrade option. Sometimes it is possible to just replace the single card in a few years with another single card, altho it is conceivable that the better option in a few years is to just buy another 6950/6970.