As long as the 500W PSU physically fits, then you'd have more than enough available current to run plenty of accessories or extra drives if you wanted, and would certainly be understressed in your present machine.

Very few power supplies say 240V on their specifications, with the absolute vast majority stating either 220V or 230V. The point is, it doesn't really matter. Being switched mode power supplies, they can tolerate quite a variation in input voltage so as long as your 500W supply is well ventilated and free of dust (a problem with older machines), then go right ahead and use it if you wish. 🙂