Since you want to upgrade "quickly and easily" I think Hunter has given a pretty good build, but I would like to take my own stab at a more crossfire-ready system.
I'll do it a bit differently. I'll make a strong base but you may end up upgrading the CPU a bit earlier:
Case and PSU
If you get a really good case and a really good PSU, you won't need to replace them for many years. The 550VX is a great PSU, but it limits what you can do too much.
PC P&C 750W
Cooler Master 690
CPU and MB
An X3 720 will be just fine to start. It comes in a combo deal with the above board, at a HUGE discount. I recommend you grab it before they figure out the error.
X3 720 and Asus 790GX
For 5 bucks more you can get some faster DDR3. This build won't use all the speed, but your next build might:
OCZ 1600Mhz 4GB
Get the HD referenced and an optical drive.
Now let's get the top-ranked 22" gaming monitor...
LG L227WTG-PF Black 22" 2ms Widescreen LCD Free shipping. Nice. I think we're about $800.
Sapphire 4870 1GB
There you go, you should come in right around 1K after shipping and before any rebates.
You can take this build and plug in a future AM3 CPU. You are also completely ready to drop in a second 4870 1GB for some killer crossfire. The case has all the fans you need.
You can also ditch the CPU cooler and get an aftermarket cooler:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185093
When you are ready to overclock.