OK.. I made a quick build for you of which I'll post a screenshot of below. It is for your Antec 900 case, an 800 watt Rosewill power supply (which actually has some really good reviews and is apparently a re-badged power supply of a better brand), ATI5850 Video Card, 6GB DDR3 (1600) ram, a decent ASUS motherboard, a decent 1TB hard drive, and basic DVD burner. The price comes to just below $1100.
This does not include Windows or a Monitor.. or a keyboard/mouse, etc...
Now, you can go with a sweet C2D (or C2Q) setup and save a few hundred bucks in which you can put into a better video card... or two... but if you're dead-set on the Antec 900 case and an I7 processor, you're going to have to compromise with a lesser video card (not that the 5850 is bad...)
Of course, you're going to go overbudget if you want SSD/RAID, blu-ray, sound card, SLI/Crossfire, or anything else fancy in the I7 example with your budget. Basically you're going to have to choose between an 'entry level' i7, or a 'maxed out' C2D...