Xfx GeForce 9800GT

Probably not, they consume aroune 130-140watts I think like 150 under full load but I might be wrong, it would probably work but it would not be safe at all. I tried running a 9800GT EE on a 350WATT and it worked out fine so mabey a 9800GT EE will work, its a energy saving version of the 9800GT that doesn't need a 6pin pci-e connector.

I would replace the power supply, they are so cheap now adays.

the ATI 5670 will give you similar performance to the 9800GT EE, similar price, and draws very little power so a 300w PSU can handle it

No it will not work. Period. Way to much at stake even if the computer does boot up.

Upgrade your PSU, or your entire rig.

A 300w PSU never can handle HD 5670....where did u gate that one from..?