You do tend to pay a premium for pre-built systems and more again for big brands like Alienware/Dell, but you can sometimes get good deals (i.e. certain combinations of offers and voucher codes) which bring them close. The main advantage is warranty, in the event of a problem they will send an engineer to your house where as if you build a system yourself it's down to you to figure out what's wrong and get it fixed (individual components still have warranties of course). The main disadvantage is the lack of customisation options, choices of cooling or specific components, no overclocking etc.
In your case @
Oona a pre-built system with a custom configuration from a smaller supplier might be a good compromise. This one from Overclockers UK looks pretty good (and they do international delivery), not too expensive and you get a decent range of choices for everything:
http://www.overclockers.co.uk/syscon_int.php?prodid=FS-387-OE
Not sure how much you're looking to spend and you would want to tweak it to your liking, but on the higher end as an example config this would be an awesome gaming PC for £1,570 (inc. VAT):
Cooler Master Silencio 650 Silent Tower Case - Black
Stage 1 Intel: Overclock of CPU - 4.2GHz Overclock - Haswell K Edition Processors Only
Intel Core i5-4670K 3.40GHz (Haswell) Socket LGA1150 Processor - Retail
BeQuiet Dark Rock Pro 2 CPU Cooler
Gigabyte Z87X-UD3H Intel Z87 (Socket 1150) DDR3 ATX Motherboard
TeamGroup Vulcan ORANGE 8GB (2x4GB) DDR3 PC3-19200C10 2400MHz Dual Channel Kit
Gigabyte GeForce GTX 780 WindForce 3x OC 3072MB GDDR5 PCI-Express Graphics Card
Samsung 120GB SSD 840 SATA 6Gb/s Basic - (MZ-7TD120BW)
Western Digital Caviar Blue 1TB SATA 6Gb/s 64MB Cache WD10EZEX - OEM ** Single Platter ** HDD
OcUK 20x DVD±RW SATA ReWriter (Black) - OEM
Creative Sound Blaster Z High Performance Gaming Sound Card - OEM
Corsair AX760i Digital ATX '80 Plus Platinum' Modular Power Supply
Case Mods Not Selected
Networking Not Selected
Keyboard Not Selected
Mouse Not Selected
System Build Fan - Noiseblocker BlackSilent Pro Fan PLPS - 120mm PWM
Monitor Not Selected
Speakers Not Selected
Game Controller Not Selected
Standard Build Systems - Approximately 5-7 working days
In terms of noise you've got:
Some sound proofing and a total of 11 fans; 4 x 120mm case fans with a max of 1200rpm, 3 x 140mm low rpm fans for the PSU and CPU, and 3 x 75mm fans for the graphics card. Nothing too fast and whiney (which I personally find the most annoying) and overall it should be reasonably quiet while maintaining a very respectable cooling performance. Also that AX760i is the best PSU on the market, something worth paying extra for particularly if you want a quiet PC as more efficiency = less heat to dissipate in the first place, its fan doesn't even come on <~150w load and even at 600w it only spins at ~1000rpm (and this is with 40c ambient:
http://www.techpowerup.com/reviews/Corsair/AX760i/6.html).
In terms of performance you've got:
4 of the fastest CPU cores around (~10% quicker than an i7 2600k and ~5% quicker than an i7 3770k at the same clock speed) running at 4.2ghz, which is pretty damn fast and a perfectly reasonable overclock that you don't have to worry about given the awesome components involved (i.e. best PSU, best motherboard VRMs and good cooling). You haven't got hyperthreading (the only difference between this and the 4770k), but that's useless for games anyway (as is more cores). You've also got the fastest RAM around (speeds tend to go down with denser chips, more slots used and higher overclocks), only 8GB but that's fine (and cheap!). Finally you have one most beastly graphics card, which let's face it is the most important component for 3D games!
The soundcard is something often overlooked these days (even that £2.2k Alienware doesn't bother!) but definitely worth having in my opinion; mostly just because onboard sound really sucks (even in high end motherboards like Asus ROG with SupremeFX add-in cards), but also because they have their own processors which take a load off the CPU.
