I've got about 1300-1400 to spend.
In US dollars? With that budget, choosing between Intel and AMD are a non-issue.
The i7 is clearly more superior in terms of performance. But the Quad-core i5 will give you the extra budget for the other components. The Phenom Quad-core will give you even more spare budget for that matter. Gaming rarely requires more than 4 cores, so I don't think the Thuban will be a good option for your profile.
Does that 1400 includes everything like, say, a 24" gaming monitor, accessories, etc? Also, are you planning on using dual GPU setup (crossfire or SLI)?