Alright alright... enough e-penis waving.
Nice rigs though.
If you're trying to build a good gaming rig, you have to cover both the CPU and video card bases those are the 2 key area. If either part isn't that good it will kill the gaming experience.
That said, if you don't know a Core duo from an i5, i7, AM2, phenom... you're best off buying something pre-built unless you want to do it for fun (because you probably won't save money).
Basically, if you want to build a computer I'd go to the website mentioned above and look @ those rigs, and do some research on the parts.
If you want to see high end CPU's go here: http://www.cpubenchmark.net/high_end_cpus.html
pick some out of that list and go find them on NewEgg figure out how much you want to spend. Repeat the process for the video card: http://www.videocardbenchmark.net/
Keep in mind that the CPU/Video cards at the top of that chart will often be $1000+ on their own.
Does that 1500 include monitor(s)?
Here's what you should be aiming for for a decent gaming rig in that price range.
Here's a list of components that you'll need and what you should expect to spend:
CPU: $250 - 400
Motherboard: $150 - 300
Video Card: $200 - 400
RAM - $100 - 200
Sound card: ~$80 (might have a decent one on board)
Power Supply: $80 - 150
Tower: $75 - 150 (with power supply 200ish)
Hard Disk: $50 - 100
Peripherals (burner, card readers, speakers, etc): $50 - 200
If you're looking to do it for fun and want to do the leg work... post up what you're thinking and I'll help you out by making sure you're not making obvious mistakes/make recommendations. Otherwise, I'd probably take comeradandrew's advice and find a pre-built rig that's in your price range.