Why not just buy it in pieces, intstead of having to settle for a barebones kit or getting a BB kit then buying more stuff to replace the stuff in the kit? Computers are extremely easy to hook up. If you can read, you can do it. I would say, if you want a cheap gaming rig, well its not gonna be a gaming rig. What is your budget for everything? Even at $500 you can get a decent PC that will play most games, but will by no means be the best. I would say go with a C2D or maybe even a C2Q CPU. These are the CPUs I would recommend.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115037
OR
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115018
For motherboard, it is mostly about what you want to do with your CPU, and what components you wanna hook up. A buddy of mine has this motherboard, which just so happens to be on sale. Even though the board is not very recent, it still works great to OC.
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3839553&CatId=2541
For GPU, I would stick with NVIDIA chipsets. Most people highly recommend the 8800 series, but I have never tried one. I personally would go with this one.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500019
As for memory, I have this and it works fine and overclocks to about 1000mhz, and it is extremely cheap. After rebate, its basically free. (ONLY 8 BUCKS!!)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820609027
For hard drive, I guess its mostly up to you. It depends on how much you plan on storing. Some people get a fast small drive for their OS and then a big slow drive for their storage, but I dont find this necessary when on a tight budget. I have this HDD and it works great.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218
As for PSU, dont go cheap on this either. Go with this one I would say. Just in case you ever wanna grab another GPU and run SLI
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182032
What am I missing? Case is about it. Just get one you like that will house all your components. Same goes for any add-ons like DVD/CD/blah blah blah. Go with what you want.