I want to get the Gainward 9800GT 512MB "Green Edition" graphics card (URL: http://www.gainward.com/main/vgapro.php?id=213) but the spec sheet says I need a PSU of 400W but I only have a 300W PSU in my HP a6305.uk PC. Does it have to be a 400W PSU or can I get away with 300W. Thank you.
It may run with a 300W PSU, but that is of course assuming that PSU can sustain 200+ W while gaming for more than an hour without crying for it's mommy. Unless it's a high quality unit, I doubt it will survive very long without blowing out, that's if it doesn't blow the first time you try and play a game anyway.
For you weak PSU, the best you can do is a GT220 and maybe a GT240. The 4670 and the 5670 use more power than the GT240 so I wouldn't consider them with that weak PSU.
There is only like 10 watt difference on load between a gt240 and 5670 or 9800gt green. Max power draw from pci-e is 75 watt anyway, seems like it won't make much difference.
I have a 300 watt as well, lite-on power supply in acer prebuilt. It says up to 20A on 12V rails, hopefully it will power a 5670 okay. I think it will be okay since my core 2 quad Q8300 doesn't draw too much power.