Need opinions on a Graphics Card for a brand new system

Hello Everyone,

With the recent release of the LGA 1155 socket and Sandy Bridge Processors I thought it would be a good time right now to build a new gaming system. I have picked out most other major components but I'm stuck deciding on a graphics card. There's just too much I don't know and won't add-up in my books and I can't seem to find the information I need anywhere. I've allocated $150 - $250 for the graphics card.

Here's the list of crucial components I have picked out so far:

Cooler Master High Air Flow (HAF) 922 Mid-Tower Case
Ultra X4 750W PSU (SLI and CrossFireX Certified)
ASUS P8P67 Pro Intel P67 Motherboard
Intel i5-2500K 3.3GHz QuadCore Processor

This rig will be mostly used to play StarCraft II and Guild Wars 2 (when it's released) so it won't require a super high-end configuration. I play at 1920x1080 monitor resolution. I do want to overclock in the future and/or buy additional CrossFireX/SLI cards if necessary.

The biggest confusion I have with the graphics card is the difference between manufacturers of a particular series (eg. XFX vs. Visiontek). What makes a Visiontek 5870 so special that it feels the need to charge an extra 200 dollars compared to an XFX 5870?

Thanks in advance for any help provided