Well obviously 2k and self build will get the absolute dogs bollocks system but I myself am a fan of pre built simply for the out the box usability, and getting a tried and tested system with a warranty. Yes building a custom all the parts have the same warranty, but they dont cover you when you short the graphics card because you didnt earth yourself properly.
So for £2000 you really do have some fantastic options. If you want something that looks cool and designed for gaming stuff like alienware and Monkey systems do some cool looking stuff, but personall I would recommend something like a top of the range fujitsu siemens Scaleo PC. For around £1000 you will get all the latest processors, graphics card etc, but here's the beauty:
You can buy the system without monitor and with dirt cheap peripherals, and instead buy seperately a Logitech G5 laser mouse, or G7 if you want wireless, and get yourself the Mac Daddy of keyboards in the shape of the Logitech G15. It's wired so no pissing about with batteries, and comes with backlit keys so you dont need the lights on when playing CS etc, an LCD screen that has user programmable scripting language, already there are many custom made apps available for it that do various things from controlling itunes to displaying chat lines as they appear in Irc! Finally it comes with a set of "G" keys. These are like a second numeric keyboard to the left of the keyboard, and are fully programmable and recordable, and compatible with all games including Ultima Online etc. So people are actually recording length macros with it.
So that would get you the very best peripherals, and then with the rest of the money get the dogs bollocks of TFT monitors. A 21" monster or something.
Or forget gaming and get one of these:
http://www.fujitsu-siemens.com/Resource ... 038491.jpg
Ive had a play with Windows Media Centre enabled homes and have to say it's fucking cool, with things like wireless streaming of media files to your various consumer electronics units like your LCD TV, Amplifier for music etc. Better yet Xbox 360 is specifically designed to hook up to WMC and stream files to your TV in HD format.
Here's the spec sheet for the top end Scaleo PC:
http://www.fujitsu-siemens.com/Resource ... 790482.pdf
They are quite sexy looking and come with front access USB and multi memory card reaers. The best thing is how customisable they are, you can specify Intel or AMD CPUs, get every type of optical drive or how many you want right upto dual layer dvd writers, dual layers are worth getting because it means you can fit 4 hours of DVD quality film onto one disc so is essential for films like lord of the rings, or fitting four tv series episodes on one disc.
Specifiable graphics cards (PCI E) and latest soundblaster audigy 7.1 or whatever it is.
Of course for hardcore FPS gaming this might not be what you want, because stuff like alienware comes with dual core motherboards and processors but I prefer stuff like the Fujitsu for flexibility, after all we all do more than gaming with our PCs.
Ultimately up to you though!
If you go the Alienware or Monkey bear in mind you are paying a lot of money just for style, they are certainly not value for money machines. But with a budget of £2000 you can afford their kit.