$900 will get you a monster that will last you many years. Currently the i5 2500Ks give the best bang for your buck, but given your guidelines I'd say the i3 2100 is suitable.
For this mockup build, I'm assuming you wont be overclocking. If you are, you should go with the now tried and true 2500K with a p67 board. Because SSD's arent really worth it, theres no need to go for a z68 board.
RAM: get a cheap 4GB DDR3 1333mhz kit.
As for the motherboard, choose an h67 from the Asus lineup. Again, if youre going with a 2500k, go for the p67's.
Power supply: Go with a PSU around 550W with 80+ certification. Refer to this list for a rough idea of which brand to go with:
http://www.eggxpert.com/forums/1/323050/ShowThread.aspx
About the 8600GT, itll work with the board as the standard connections are all the same. Look for AMD's new 7000 series cards (Southern islands) later this yr for when you're upgrading the GPU.
If you want a HDD, Samsung Spinpoint F3's are very good drives. I saw one FS in our buy/sell section too.
Case: I havent paid much attention to lower end cases, but the CM HAF 912 is really popular given its airflow, user friendliness...etc etc.
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