Don't bother with Geek Squad, they'll likely eat up 1/4 of your budget just throwing stuff together half-assed and wanting you to buy 5 different protection plans for a extra $19.99! Ok, maybe not 5, but they prey on people that don't know any better and it's best to avoid them at all costs unless you can haggle your way into having them put it together for like $30-40 and make sure it boots before you walk out of there.
Putting it together yourself is still the best option and it feels damn good when you get it working too. It's a simple matter of plugging in wires and hooking things up and screwing things in, with the most daunting task being gently putting the CPU into it's socket.
When it comes to WoW, the video card isn't actually the top priority, as it heavily relies on the processor.
Your looking at a Intel build if your playing WoW, something like this:
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i3-4130 3.4GHz Dual-Core Processor ($119.99 @ TigerDirect)
Motherboard: MSI H81M-P33 Micro ATX LGA1150 Motherboard ($39.99 @ Newegg)
Memory: A-Data XPG Gaming Series 8GB (2 x 4GB) DDR3-1600 Memory ($62.99 @ Newegg)
Video Card: MSI GeForce GTX 650 Ti Boost 1GB Video Card ($130.98 @ SuperBiiz)
Power Supply: Corsair CX 430W 80+ Bronze Certified Semi-Modular ATX Power Supply ($29.99 @ Newegg)
Optical Drive: LG GH24NS95 DVD/CD Writer ($21.98 @ OutletPC)
Total: $405.92
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-27 05:35 EST-0500)
At the bottom edge of your budget and should be able to handle playing WoW just fine, though there's room enough for some more upgrades as well.