I bought a Dell XPS with an i7 920, 6GB of RAM, a Nvidia 220 graphics card (I think), and a 640GB HDD for about $700 a few months ago. You should be able to get a similar deal for $600 or less if you watch and wait a bit.
I also bought an older model Dell XPS with a quad core Intel 8200 on eBay for $500 (new, built to order with Win7, from Dell.)
Things to look at are available drives/bays, slots for expansion cards, enough slots to get to 12GB of DDR3 RAM (6 slots) and later 24GB of RAM (when 4GB chips are a bit cheaper), and a good sized case.
You can easily and cheaply add HDD's (1 TB WD was $65 at Newegg recently), RAM, and even upgrade the power supply ev entually if you stuff the box full.
Don't obsess too much. Maybe start a small spreadsheet and track prices as a memory aid to remember the best deals?
Watch the "Bing" cash back deals too. Dell had 15% cash back, HP 25% cash back at specific times. Also look at techbargains.com