Where will you be gaming? (If it's with a club, check with their rules) If it's for plinking it's totally up to you.
For me, anything over a 150 is too hot. Our limit is 475 with .2's on a BA.
Another thing to remember, is at those higher speeds, BB's will lose more stability. Going too hot gives you erratic shot patterns with spherical projectiles. I've heard of long range kills with only about 380.
As well, if you're going above 500 you'll want to look at upgrading your trigger sears, which aren't exactly cheap.
I'd reccomend getting something like the 130 for now, and when are sure you're good with picking your shots (hit areas that aren't dangerous, but they can feel it, like the shoulder, rump, etc), and good with maintaining a safe engagement distance, than I'd go up to something like a 150.
For spring power, you'll really want to talk with the people you'll be gaming with. If it's an established group they'll have a set of rules they generally stick with, and if it's a new group of guys, talk it over with them to get an idea of what you're limits will be, and see what other groups do.