Here's a build for you. Since you're gaming, especially with an emphasis on games that can't utilize more than 4 cores, you can save money without losing performance by going for an Intel i3 (unclockable, but performance equivalent to overclocked AMD phenom IIs for less).
CPU - i3-2120 + DVD burner combo ($131)
Cheap H61 mobo ($44 AMIR)
RAM - 2x4gb 1333mhz ($39)
GPU: 6870 ($140 +promo code and after $30 rebate)
Case: Coolermaster 912 ($53)
PSU: Rosewill Green 630w ($50 after promo code)
Overkill wattage, but no other good sales, so overkill it is.
SSD - Crucial m4 64gb for $90 (recommended) or 128GB for $125
Normally I wouldn't recommend an SSD at this budget. But for MMOs it actually provides some in-game benefits. You'll see less loading time for new textures entering your view. Most notable in games like WoW when meeting a large band of enemy units in Alterac Valley or outside the auction house in busy cities like Orgrimmar or Ironforge. The SSD you get should be big enough to fit your MMOs, most used programs, and possibly your OS.
HDD: 500GB Seagate Barracuda ($70)
Total: $617 w/64gb SSD
Note: this is a little over budget & will cost more up front because of the ~$40 rebates. But it will be a beastly MMO computer, with an SSD & a 6870 as a video card. Drop the SSD if you need to meet strict budgetary concerns. Or you can consider dropping to a 6850 video card for $130:
Edit: I just realized you need a monitor too. You can free up more money for a monitor by dropping the SSD and going to the 6850. But at that point you might consider waiting to see the performance/price of the next generation AMD APUs (not released yet, dunno when, ask the AMD-focused posters).Edited by MisterFred - 5/6/12 at 8:31pm