New PC, help! with upgrades

Hi there guys Im quite into computer games, more than I was say last year, Ive only ever played counter-strike and my current compaq 1.86 dual core could handle that no problem, but games like farcry >>> it chokes on.

So I want to buy a new pc. Its an ADVENT T9509, I know someone who can get me the base unit for about £400. The specs of this system are:

• Intel Core 2 Quad Q6600
  2GB RAM
  320GB Hard Drive
  Nvidia Geforce 8300LE

So I know the graphics card isnt gaming material but I plan to upgrade that to an 8800GT

Now this is where my problem is, I know these specs are rather good for the money and when I upgrade the graphics to the GT it should easily be able to handle Bioshock. The only problem is that this system only has a 350 watt PSU which is definately not enough

I know which PSU i want to buy, I read on another forum about someone who had this same pc and upgraded the psu to a 550 watt and a 8800GT and he sayd it ran like a dream. So this is the upgrades I plan to buy

AKASA 550w PSU AK-P550FH Quiet ATX12V/EPS12V v2.01 Sli £30.00
Evercool SB-F2 Dual PCI System Exhaust Fan Blower £4.50
Arctic Cooling Freezer 7 Pro £10.00
Evercool HD-117 3.5" Hard drive Cooler HDD £7.00

Ok so those upgrades seem pretty good, to me that is, but I dont have a clue what im talking about, im a noob and have never upgraded a pc.

So can you tell me if these seem a good choice and is it easy to install a new PSU and all of these parts, or should I go to a specialist especially since ive never done it before.

Thank you all, much appreciated