Two recommendations:
Based on my experience with Asus, I'd recommend going with one of their competitors for your mainboard. Try the
Gigabyte UD3P as a reasonably priced alternative at $125 after rebate.
Next, spend a bit more money on the GPU and get a
GTX 260 for $160 after rebate. If you can spend it, you'd be very wise to pick up the
GTX 260 Core 216. It's worth it for the extra shader processors.
And waverrider had the right idea about cases - you can find some better options out there for about the same money, but that's a much more subjective issue, so I'll not comment. Enjoy!