My all time favorite gun is the ego's and you can pick a 06-11 up for that price range super easy, there's a lot of highend markers used that you can buy for that price range it all depends on what you like, try test shooting some different brands of guns to see what fits your playing style the best
Honestly most guns you buy from here will work but to make sure check peoples feedback and if they are bst trusted and all those sorts of things. But back to the real question...in my opinion around $650 some of the best guns you can buy are Ego 11's. You can find them around that price maybe slightly more expensive but that is probably one of the best guns you can also check out G6R's and just look around the bst and see what you find. It also depends on if you like poppits or spoolies and if you like spoolies probably one of the best spoolies would be either a Geo 2 or a Macdev Drone.
Check your local classifieds and the bst section. If you buy used obviously odds are you won't get warrantee which is a down, but if you find something solid it's well worth the chance. I picked up an up'd ego8 for 400 bucks, and am in love with it. Shoots smooth, low maintainance, efficient. But at the same time, I've seen people play very solid with a bummer of an ion. Which you could pick up for less then 200 then spend on more paint. I'm all for used markers. Save money and still get a high end gun. In the end, find something that feels good in your hands, and go play