Installing ASUS HD 5770 on HP Pavilion a6450.

I'm planning to buy an ASUS CuCore Series EAH5770 CuCore/2DI/1GD5 and i wonder if I must buy a new psu along in order to get it running.

My pc is HP Pavilion a6450.is (Specs page), which currently has 300w psu installed. So in case i have to replace the psu, which model should i get considering I'm pretty low on budget? I've been thinking about CORSAIR CMPSU-400CX 400W, but is there anything cheaper like some $20 500w models?

Another question: Will this GPU fit in my case, considering it's pretty big in comparison with the 8600gt i currently have. Here's the mobo specs.

Sorry for my bad English.