I highly doubt you would have much of an issue running DA:I better with that rig compared to what the consoles can do. At the very least, you should still be able to get 60fps at 1440p with maybe the occasional dip in the upper 50s unless the screen gets really chaotic. Watch Dogs with the mod just doesn't seem optimized at all, which is probably the reason it requires a more brute force approach, but you can still make it look better without the mod and without 3gb of Vram compared to the consoles, if only marignally. DA:I shouldn't really be nearly as taxing and I'd argue that you shouldn't really worry too much about performance unless you're trying to do something crazy like playing at 120fps @ 2160p. Bioware is using an engine that is pretty new to them, so maybe their first game using it won't be as optimized as those that follow, but I wouldn't be surprised if you could still get a relatively good looking and playing experience out of a single 580 GTX.
Since the PC version will also allow the use of a controller, you may as well just get that version, especially because you can more than likely find it cheaper online and there may eventually be some simple mods (mostly graphical) that you wouldn't get for the console versions, and you'd have something to look forward to after your next PC upgrade.
