I want to say that buiding your own system can be quite expensive, More so than a system from Dell HP etc. but it is very gratifying, and can be a very fast system. Usuall you can expect to pay between $1700 - $2700 for a very high end self built system.First you need to start with a quality motherboard. ASUS ABIT SOYO GIGABYTE MICRO are all good names, but even the best names have cheap boards, usually a good board will cost between $150 and $190. If you pay less make sure it is a steal otherwise you might not get the performance you were looking for. After you decide what board you will buy, Decide on a Case that will house your new MOBO and get one with at least a 450 watt Power Supply. Now find the right CPU that will first of all work on the board you picked, and serve you well for 2 to 3 years. Most computers are out of date within 1 years of being purchased, and thats being nice. Choosing a very fast CPU with a lot of on board level 2 cache preferably a Quad core but a dual core would do. Expect to pay around $200 for a good CPU but for a very fast CPU you could spend as much as $800 (extreme editions)(I like Intel CPU's - I have never had anything bad to say about, or had anything go wrong with them.) Do your own research for a heatsink fan combo for your CPU and use arctic silver heatsink compound for optimum heat transfer. RAM, OZC is a great namebrand. You need the fastest your board will support, and no less than 2Gigs for starters. DDR2 DDR3, Buy the fastest memory your board will support, you wont regret it later. Video, You need to buy a 3D graphics card that is a PCI-Express. Nvidia Geforce 8800 and up, no less. You'll want to spend at least $200-$300 on a descent vidio card. Hard drives. Get at least 1 Terabyte,They are getting cheaper by the day and look for names such as Western Digital, or Seagate. $150 should cover 1 of those drives oh and they should be SATA drives. DVD drives- Go ahead and get a Sony DVD Blu ray R/W combo drive. Two if you want multiple drives. These are beginning to be standard equipment on OEM systems. Buy a linksys wirless router and a Linksys Wirless PCI card for your internet connection. these should be no less that a 802.11g, but look for an 802.11n.  The video card and wirless card should be the only cards in your system. unless you want to do some tv tuning or capture, you shouldnt bog your system down with something that just gonna eat up system resources and memory addresses. Think outside the box. Most peripherals can now be bought in USB 2.0 or firewire and should be used instead of internal stuff. Get all this stuff throw it all together and you got yourself a very flashy super quick PC Put you own custom case stickers and badges to decorate it and make it your own. A system built to these standards will play any software title for at least 2 years to come. Mine does. and is still kicking.

My System:

FoxConn BTX MoBo LGA775

CPU Intel 3.4Ghz 800Mhz FSB 4Mb L2 Cache x 2 (Pentium D dual core Engineering Sample)

OZC Nvidia SLI 4Gig DDR2 PC6400 SDram

Nvidia 8600GTS 256Mb PCI-E

Sony DVD-R/W Drive 

My system is 3 years old and still kicking. Good Luck with your system If you need help or want some advice drop me a line. I will help as much as I can.