The best card is HD5970! But if you tell us your system specs we can help you find a suitable card for your system that does not cost $600-700
I dunno, 3 way GTX 480 SLI love would probably be better than a single HD5970.
In all sincerity though, to the OP's question, an ATI 5750 would probably play WoW very well. But you might not even need that depending on what resolution you play at, and what graphical settings you wish to have.
Being single threaded, and with all the addons/plugins for WoW it can tend to be CPU bound.
To be honest the best cards for the job are the 9800gt,48x0,gts250,57x0,gtx 460, and the 5850. For single instance wow a 5770 or gtx 460 is more than enough. My GTX 460 can handle 4 instances of wow at once so it is more than enough really. Got 6 accounts
Well I used to play wow on my 8600GT right up to and including WotLK on a 2.4ghz dual core so... you don't need much.
Used to is a key word. When I first started playing wow it was a 9800pro aiw and later a evga 7800gs co when it had little over heard but with recent patches it can take a lot for it's self and run with it like every thing else these days.
Keep in mind, a 5770 may play almost maxed, but you'll have to turn shadows off, or only at the lowest quality. If you don't mind that, it'll get the job done. If you want to play at the highest settings, including shadows maxed, you'll need at least a 480. If you don't mind turning shadows down 1 notch, a 5870 will work.
This if you play at 1920x1200.
The good thing is, WoW does look pretty decent even when it's not maxed, but I do find shadows at one notch below max is needed for maximum enjoyment in my experience.
G.SKILL ECO Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600
Sony Optiarc DVD Burner with LightScribe Black SATA Model AD-7241S-0B LightScribe
PLEXTOR Black Blu-ray Disc Burner SATA
Antec Max CPU Cooler 92mm CPU Cooler
Western Digital VelociRaptor WD3000HLFS 300GB 10000 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive
Microsoft Windows 7 Professional 64-bit
EVGA 512-P3-1154-TR GeForce GTS 250 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
MSI 790FX-GD70 AM3 AMD 790FX ATX AMD Motherboard
KINGWIN Mach 1 ABT-1000MA1S 1000W ATX / BTX SLI Certified CrossFire Ready 80 PLUS BRONZE Certified Modular
AMD Phenom II X4 965 Black Edition Deneb 3.4GHz Socket AM3 125W Quad-Core Processor
Based on CT´s link above you are fine where you are at with the GTS250 if your resolution is 1680x1050 or lower. But a GTX460 1 GB would be a nice upgrade especially if you are playing at 1080p or higher.