A#12: Your opponent has donked into you on the flop, so you have to be a little wary. Most good players will (correctly) check to the PF raiser, so donking should raise some eyebrows. This player, however is said to be "weak," which implies he doesn't really know what he's doing. He presumably likes his hand, so he's leading out on the flop. This could mean he has a set, a middle pair, or top pair. It could also be a blocking bet with some kind of draw. With some of these you're way behind, and with others you're way ahead. What to do?
The answer to this lies in the concept of pot commitment. Your opponent only had $400 when the hand started. Your stack is bigger than this, but the smaller stack size sets the "effective" stack size of the hand. Said another way, you both effectively had $400 stacks at the beginning of the hand.
After the preflop raising, the pot size on the flop prior to any action is: ($5 + $10 + $140 + $140) = $295. Your SPR (Stack-to-Pot Ratio) is therefore $260 / $295 = ~0.9. This is very small, and therefore you were pot committed to the hand the minute you put the raise in preflop. Said another way, regardless of the villain's donk on the flop (or even the flop texture itself (unless it's REALLY hideous)), you were committed to getting all the money into the middle. Nothing has changed, so it's time to re-raise and get the money in.
The book Professional No-Limit Hold'em by Miller, et al., has a really good discussion on this concept of SPR and commitment. If you haven't yet read this book, I'd suggest getting a copy and boning up on the theory. Figuring out whether you're going to be committed or not before you act in a hand is essential to winning poker. Professional players make these SPR calculations and decisions before deciding on a line, and so should you. The goal is to make poker decisions easy after the flop, and you start this process by looking at your commitment threshold and planning before the flop.
Ergo, the answer here is: Raise.
All-in for now....