My first gaming computer cost about $920, and it can still run just about everything on max settings. With hardware prices coming down so far, you could probably build an ok system for near $600, you just need to know where to get your parts. Having the hard drive already helps a lot. newegg.com has never done me wrong, but if you're confident in the quality of the parts, buying from ebay shouldn't give you too much trouble as long as you verify the part spex. Great prices and very well organized site with a trustworthy rating system. Click "shop all stores" and go into "computer hardware", then find all of the individual parts that you need.
To build a gaming computer you need the following:
Motherboard
Processor + heatsync
Power Supply
RAM
Hard Drive
Disk Drive
Graphics Card
Case
-I've built computers in the past around processors. For a Gaming computer I recommend at least a 3 core processor at 2.8ghz+. The processor and the graphics card are the two parts you should spend the most on. A processor will almost always come with a heat sync, but if you want to overclock your processor you may want to add a custom cooling unit (I do not generally recommend this). If you wish to run either mac or Linux on your computer along with windows, I recommend an Intel processor, but if you only want to run windows, I recommend AMD. The only real benefit you would have in spending more on an Intel would be compatibility with Unix based OSs, which are useless for gaming.
-Next, I find a motherboard that has the appropriate processor socket that lists support for your processor Wattage and type. There are usually too many components to a motherboard to be too specific, but just make sure it's compatible with what you need and has a good rating for its price.
- You may find a motherboard you like and choose RAM to go with it, or find RAM and choose a motherboard that is compatible with your processor. The higher the number of the ram's DDR, the faster it'll go. Standard at this point is now DDR3, but some computer run as fast as DDR5 or more. For a gaming computer I recommend no less than 6GB of RAM, but for most games RAM requirement is not very high.
- The Graphics card is the most crucial bottleneck of your gaming computer. They're very complicated and have many specs. You will probably spend more on the card than on your processor. My graphics card is 1GB 256bit DDR3 NVidia with 700MHz core clock speed and I'm able to run most new games on max graphics settings. Normally you can trust the reviews of how good a card is, but you'll generally get what you pay for. Just make sure you have the right PCI port to run the card you choose. Also make sure you get one with the outputs you need, HDMI, VGA, or DVI.
- Power supplies can be complicated, but newegg has a great feature that calculates the wattage you'll need for your computer. It's under "Computer Hardware", the last option on the far right, "Power Supply Wattage Calculator". Fill out the info, get your wattage, find a high rated power supply with that wattage. easy-peasy
- Don't spend too much on a case, just make sure your stuff will fit. parts are usually the right size, you just need the right number of ports for hard drives and disk drives. If you have an existing case you want to use, go for it.
- Get a disk drive, a burner might be nice, not a big deal.
- As far as the hard drive goes, just format the drive to wipe all the data and it shouldn't give you too many problems. You might have to find a SATA cable to hook it up to a new motherboard if you don't have one.
- Most new hardware will just run 64 bit by default, but you'll want to verify that your processor will run it. If it's 3 cores or more, it probably will.
Good luck!