Upgrading from Radeon 5450

Hello everyone,

I currently have a dell inspiron 580 computer, which is about 2 years old.
These are the specs:
64 bit Intel core i3 CPU 550 3,20 GHz
6 GB of ram
ATI HD Radeon 5450 1GB
1 TB of hard disk

I have opened the case and saw that I a power supply of max 300 Watt. I don't know if this is true, because I found out that the recommended power supply of the Radeon 5450 is 400 watt.

I want to play games like Civilization V, GTA V and Just Cause 2. Those games, however, are too heavy for my video card and I don't get the frames I want. (usually beneath 20) So I want to upgrade my videocard, but I am stuck on which videocards are possible for my computer. I know I have PCI slot, so that's good, but I don't know if the upgrades will work with my power supply.

I looked at an upgrade such as a Radeon HD 5770 or in the 5000 serie. My question is, can I upgrade to these videocards or maybe 6000 series with these specs or do I need to just buy a whole new system to play these games without lag?

Thanks for the support!

David