Follow this guide and you'll be fine.The big secret to building your own computer is to log onto newegg, sort everything by "most reviews," and buy all the highest rated stuff that fits into your budget and actually fits together (e.g. no i5 processors in an AMD2+ board). When you get to the bit about graphics cards, read the reviews in a little more detail to figure out what card you should be buying. If your goal is to run Murder Simulator 2012 on high settings, and there's 3 reviews saying card 100 does it on high settings, then you don't need card 300. Everyone I know in real life builds computers this way, and having built several computers this way myself, I promise you'll come out on top and under-budget every time, assuming your expectations are realistic. You can probably build a computer that would run Oblivion at maximum settings for less than $600. In fact, my new computer which runs most stuff I've thrown at it on max settings (including oblivion) only cost $700 (although the most intensive thing I've thrown at it is GTA4 so far, and I'm not including the widescreen monitor I bought with it in the price, I'm assuming you're reusing a monitor from an old pc).
If you're really picky about getting the absolute best stuff for the lowest price, realize someone else has already done all the work for you and just follow a guide. Off the top of my head:
http://www.neogaf.com/forum/showthread.php?t=417014http://arstechnica.com/gadgets/guides/2011/03/ars-system-guide-march-2011-edition.arsYou only really need to brown townyze all the parts is if you really love shopping for and speccing computers, or you need to design a specific computer that meets certain power consumption requirements, size constraints, etc.
