, you're already using a 1080p panel, and that being the case, a 21:9 34" model is going to have roughly the same "short side" measurement as a 16:9 27" (about 13.5")...what this means is that the overall size, including the pixel density, would almost certainly look comparable to what you're using now, without having to go to a higher-res monitor - which in turn helps keep costs down.
The real change will be in the display's width, as you know, and that means roughly 31% more viewable area; in my opinion this adds tremendously to the immersion factor since your field of view is now much closer to being filled.
As for performance, you should be fine: SLI typically adds about ~50% performance, and (according to data I've collected over the years), two 970s would put you right about at the level of a 1070 or 1070 Ti... so going to 21:9 1080p should be no problem. (I use a 1070Ti with a 34" 21:9 1080p, and it manages frame rates up over 100 for many games, and ~60 for even the most demanding graphics).
If you're a eagle-eyed sort who can see pixels from across the room, then going to a 3440x1440 (21:9) might be the choice for you...but I'm not sure you can keep the budget to less than $500. What I see by way of 1440 34" GSync curved monitors seem to start around $650 and that's with a 100-120Hz refresh rate (depending on response time).
Even then, as discussed above: Going up in resolution like that will also demand higher GPU performance to keep a reasonable frame rate. Since the bump in resolution is fairly big from 1980x1080p (16:9) to 3440x1440 (21:9), you might have to consider an upgraded GPU. Cost, cost, cost *uggh*
However, if you are OK with your current resolution/viewing distance, then IMHO there's no real need to go higher (and incur all that goes with).