-Start out with a quality socket LGA 1155 motherboard, one that is SLI or CrossfireX ready. $150
-The cpu is next, intel beats AMD on its butt here, so get either a sandy or ivy bridge i5 or i7(more expensive) one. The i5s are all wonderful, but the 2500k is exceptionally popular and not outrageously priced. $220
-A good ATX case is essential, full tower is preferable if you have room, but a mid tower can work quite well with a full ATX MB also. Pick one you like with adequate cooling fan slots, and make sure you like its appearance, you'll see it every day for years to come. $150
-Next is the power supply, which must be ATX to match your MB and case. Make sure its Motherboard connector has the same # of pins as your MB so they will work. Get a decent quality one and avoid OEM ones with no brand as these will likely not deliver their full wattage. A single 12v rail is preferable, but dual can work if the wattage is high enough, lets play it safe if you want dual gpus and get a 700 watt one. Raidmax makes a good modular (detachable cables) one for ~$70
-RAM is RAM, you'll need around 8 Gb, Ddr3 1600Mhz is the norm, so make sure your Motherboard supports it. $60
-A disc drive is not really necessary anymore, but it helps when you install your OS, want to watch DVDs, buy boxed games, etc. A good DVD drive is $20 or so, while blu ray will run ~$60-75 at the low end. Note that Blu Ray drives will work on DVDs too.
-However, a hard drive is still essential, a seagate or Western Digital 1TB runs around $75. A Solid State Drive will boost performance, but costs an order of magnitude more, so consider getting a small one to run your OS and any frequently used programs for around $100 for ~120Gb
-The almighty graphics card is next. Radeon ones are often better values, but their drivers are often outshone by Nvidias. Look for benchmarks on a card before you buy, but be sure that they are using recent drivers as AMD has been rapidly catching up in that respect. I recommend either a radeon 7950 or a GTX 670, depending on your preferred brand. Note that the different manufacturers of the actual cards all use the same gpu chip (eg. all 7950s have essentially the same chip in their heart), but may have different amounts of memory and may be clocked at different frequencies. The fans are also not all created equal, and some are better or worse at cooling and make more/less noise. A 7950 card will cost around $300 or more with special features. A gtx670 starts at around $370
-Finally there is the operating system, which is basically a choice of Windows 7, 8, or Linux. Either windows will cost around $140 for the retail version, which you want because the license can be transferred if you replace the motherboard, which is impossible with the OEM version. Linux is free, but suffers from a dearth of commercial apps, and most games won't work on it.