A fairly quick overview between a 6790 and a 560 vanilla looks like
this. Granted, there are more detailed comparisons out there but it will boil down to whether the performance premium will justify the extra cash. There's a good chance that one is sitting on your shelf for a reason though, especially for that price.
Just be aware that the demands required to provide eye candy and performance at your resolution will not require a 300$ video card. You can get roped in to spending waaaaay too much when you may never use it to its full potential.
Without knowing how well GW2 performs between a DC and a QC, especially for WvWvW PvP, (since I'm not in the beta to test myself), it's more of a assumption that a QC will provide a little extra 'oomph' when it comes to overall performance especially if GW2 is coded in such a way which can split the load between four processors. I would personally take a QC over any DC because gaming is getting better at accommodating proper QC optimization.
Best option is to really pick away at upgrades as they come on sale. That's what I always do and it's really the best way to save money for the same end performance and you can use that saved cash for something else. Chances are there will also be some more 'official' preview benchmarks released to give a better idea of what's what too.
---------- Post added 2012-07-08 at 11:57 PM ----------
Oh, one thing I forgot to mention is that the Hateway case could be too small for the PSU too so you may need to factor in a new case as well for like 40 - 50$.