The most basic upgrade is main studs and caps. I run Program steel caps at around 550 HP.

Current thinking, I believe, is aluminum caps are even better than steel as they absorb some of the shock load.

The old "rule of thumb" is 600 HP for long term durability. Plenty of engines exceed that... and some that do break. There are lots of other variables. B vs RB, individual blocks, piston weight and especially detonation. Personally, I'd feel OK with the main studs and aluminum caps at 650 HP as long as it is not a continuous running endurance type engine. Also use extra care on the tune... not too much timing and not lean.