New gaming rig

Hi all. My first post and I am looking for some advice from you all for my new gaming rig. Have a 24 inch monitor which I would like to use to its full potential.

I wonder if someone could advise me of the following.

I am in the UK, by the way!

APPROXIMATE PURCHASE DATE: Before end October 2009

BUDGET RANGE: £900 ($1300?)

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: gaming, surfing the internet, watching movies

PARTS NOT REQUIRED: keyboard, mouse, monitor, speakers, OS, case

PREFERRED WEBSITE FOR PARTS: www.scan.co.uk

COUNTRY OF ORIGIN: UK

PARTS PREFERENCES: open to suggestion

OVERCLOCKING: Maybe

SLI OR CROSSFIRE: No

MONITOR RESOLUTION: 1920x1200

ADDITIONAL COMMENTS: I would like a quiet PC that will play the most demanding of games and last me through the next 3 years. I will spend no more than £250 ($320?) on a GPU. Case is an Akasa Eclipse 2 and it has 2 x Nexus D12SL 120mm quiet fans in it and an Akasa fan controller. Looking at i5 processor or equivalent. No interest in water cooling but don't want loud fans. No interest in Blueray and would like 1TB of storage.

OK.

I guess I was being lazy just asking for opinions so I've done a bit of research and have come up with the following:

ATi HD 5850, PCI-E 2.0, 4000MHz GDDR5, GPU 725MHz, 1440 Cores
4GB (2x2GB) Corsair Dominator GT DDR3 PC3-12800
Asus P7P55D deluxe
Intel Core i5-750 Processor (Socket 1156)
Corsair 750HX PSU
1TB Samsung SpinPoint F3 HD103SJ Hard Disk Drive (would prefer this over the WD Caviar Black I think)
Creative SoundBlaster X-FI Xtreme Gamer PCI 7.1Ch
Corsair CWCH50-1 Hydro Series H50 High-performance CPU Cooler
SATA LG DH16NS10.AUAA10B 16x Internal DVDROM

I wonder if any of you could give me your thoughts; in particular on the GPU, the PSU and the CPU cooling option (there is very little choice for 1156 coolers it seems).

Having done a bit of looking around, the performance of the ATi 5850 seems to offer me best bang for the buck and offers a degree of futureproofing with DX11, so, after a long love affair with Nvidia I hope I am doing the right thing moving away from them and towards ATi.

My other question surrounds the RAM. I have gone for an expensive option with the Dominator GT and wonder if there may be something better out there, more suited to gaming. Finally, is 4GB of great RAM better than 8GB of good RAM?.