I am using i5 with 4GB RAM and I am quite comfortable with it. I don't have to switch to i7 unless this one breaks.
At $3000, you can already get a high-end PC, not to mention you'll gonna build it yourself. I suggest 7200RPM HD for faster data read-write. Maybe a Gigabyte mother board. 4GD to 8GB RAM. I hope this helps.