repost - Help upgrading PC needed

Currently I am running a P4 2GHz, 512mb Ram and a GEForce FX5600. I was hoping to upgrade to a 2.8GHz processor but have found my motherboard is maxed out for its revision.

I am looking for a PC upgrade that is under £500 but will run games like Half Life 2 comfortably on mid settings.

I have seen these items :-

512Mb PC3200 (400mhz) Dimm X 2 = £120
P4 3.4ghz (800FSB) = £235
Intel D865PERL (s478) = £70
Geforce 6600 GT (128Ram) = £145

total = £570

Now I know these components are cheaper if I shop around buy my vendor has agreed to install them all for free. Could I ask if these parts are the minimum I need or could I cut back to keep inside my £500 (I can go over but would prefer not to)? Is there anything in my current setup that will be ok for now - graphics card for example.

Lastly I am assuming my 300 watt power supply is not good enough anymore - am I correct?

Many thanks for any help you can offer