Upgrading Gateway GT5432 PSU

Hey everyone,
I've been researching what I should upgrade my video card to and I found an interesting choice:
RADEON HD 3870
So, I checked the PSU requirements and found out I will be needing to upgrade my PSU from a 300W to 450W+
I don't really know much about power supplies, so I am concerned on what type I should be looking for: microATX or ATX? Will the ATX PSU fit in my microATX case? Which specific brand?

You can find my computers component list here:
http://support.gateway.com/s/PC/R/ [...] 4cl3.shtml

Any information on this subject would be extremely helpful!

I'm not looking to overclock anything, just wondering if I am able to switch my stock power supply with a new and improved one.

You can. But be careful. The setup on the this model is ridiculously stupid. I have the same thing and I want to upgrade my PSU as well. But as you see, it has the angled plugs on it, you'll need to find a PSU with the angeled plugs, or buy some extra cables that have the angles, as you can tell the hard drive is facing the side panel and without the angle, you wouldn't be able to close the panel.