Here is a sample build.
I am a die hard Antec and EVGA fan boy.
EVGA because of their outstanding warranty (some others offer it too) and Antec because it has never failed me.
This is without monitor, keyboard, speakers, mice, and other little things.
Case
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129154
Antec 180. Yes there is another one called 182 but not too big of a difference.
It's pretty quiet, lots of extra fans and such.
Mobo
http://www.newegg.com/Product/Product.aspx?Item=N82E16813188026
This is the motherboard of my choice if I were to build a system. It gives you 2 PCI-e slots for SLI. You will actually be baffled in a bit for my video card choice though. I don't think triple SLI will be taking major advantage for a long time plus the cost.
CPU
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115038
Intel E8200 is the cheapest 45nm (the newest tech from Intel atm) and if you want a little more speed out of it you can easily over clock it with the stock cooler safely. Until I see MOST games use Quad Core, or even fully utilize 2 cores, I will stick with just 2.
Power Supply
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371015
Antec 650 Watts. Although it is certified for dual graphics card, in all honesty, 650 isn't exactly safe. You might have to upgrade if you wish to go SLI in the future but for a current single video card. It will have extra juice running.
Memory
http://www.newegg.com/Product/Product.aspx?Item=N82E16820220227
I went with 800 instead of 1066 because there is not way too much difference in there. 2 gig is sufficient but as a gamer 4 gig is the real sweet spot. Plus, they are quiet cheap still.
Hard Drive
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152098
Samsung. They are pretty reliable drives. 500 gig should be minimum these days for anyone with a desktop. I personally have over 40 terabytes of storage between work and all the computers at home. The Raptor from WD is nice but they do get kind of expensive.
Video Card
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152098
Since you are on a budget, I'm going with 4850. And to clear up any confusion, ATI card will work on that board long as it's not crossfired. You can actually put 2 of those in there to give you 4 monitor outputs. You just simply cannot crossfire.
Maybe in the future, you will want to do 2 nVidia cards at that point, you just need to get 2 new nVidia cards for SLI.
Operating System
You can get Linux for free or buy/copy MS or use your old OS if you have the proper disks.
Without OS it came out to 874.93 before tax and shipping/handling.
Gah, have to go out on business call. Will see if I can update a bit more.