For 650, you could do alot of other things other than Nitrous.
Rockers or a cam come to mind, along with a good FWI and some exhaust. It all depends on how much work you can do yourself.
If you go nitrous, make sure it'* a wet shot. Don't go dry under any circumstances with a supercharger.
The condition of your exhaust manifolds (you better not have any hairline cracks, and probably do) as well as your fuel system MUST be PROVEN to be up to par first.
I've spent less than 650 on my performance mods to date, including gaskets.
My main beefs with Nitrous are that it doesn't tell you squat about your car. I get performance the old-fashioned way. Attention to mechanical details. The other reason is that people tend to end up getting carried away with it. I've seen my share of blown motors from it over the years.
Can you go quicker with Nitrous? Sure. But so can anyone else with a screwdriver. I want to know what my car can REALLY do. What I take to the track is what I drive every day, and I don't have to fill a bottle. Just the gas tank like everyone else.
That'* my opinion and may not be yours, but you may want to think about mechanical mods first. Then look at NO2 later.