so new PSU?
Depends on how much you are willing to spend. $100 for a 7750 is a guarantee quick fix solution. Anything more means you need to buy a PSU for a more powerful (and expensive) card so thats like over double the cost.
The 7750 will work with 377W don't worry. They've been known to work with sub-300W systems.
Crossfire is basically a feature that allows you to add another similar AMD graphics card to a second PCI-E X16/X8 slot on your motherboard. Both cards work together for better performance but you will not get close to double performance.