The reason behind building your own computer is that it's just easier to upgrade parts. I had an acer aspire and in order to upgrade any part on it, I had to get a new power supply,which meant a new case. So now that i have the new power supply,new graphics and a new case, next time i chose to upgrade, it will be the cost of a new part rather than a whole new computer.
As for gaming, it would depend on what game ( s ) you plan to run on it. Then i'd recommend going to that games forum and asking about computer specs.
I know a number of folks who have built their own systems that are cheaper than an alienware pc and can run some games at the high end specs.
http://lifehacker.com/5840963/the-best-pcs-you-can-build-for-600-and-1200
By all means, one should be able to beat the pricing on a top AlienWare. But $300 ain't gonna get you a gaming machine either.
The problem with your logic is in this fragment: cost of a new part
If you are building a top end gaming machine from scratch, 2 years from now, exactly what part do you think you are going to upgrade that is going to make the machine better than it is now?
drives don't count, they're a commodity item
memory doesn't count. You should already have the top-end of RAM you'll ever need on this technology generation (if 4GB is standard on a PC, you should have at least 16GB, and no game will use more than 8GB if 4GB is standard).
Processors are architecture and timing dependant on the motherboard. You should already have a top speed CPU, and in 2 years, the next gen CPU will have a different socket anyway, meaning you'll need a new motherboard.
Upgrading a PC in parts only happens when the existing equipment was not top of the line in the first place. If you're gimping your purchase at that level anyway, then you aren't making a gaming machine, you are dabbling.
Furthermore, any such upgrade you make is likely marginal and minimally impactful. Notice how consumer CPU speeds haven't crossed the 3GHz line in over a decade. At most, you get more cores on a single CPU. Which is nice, but ultimately, a game wasn't designed to run on a server, it's not going to get dramatic benefit from ridiculous # of cores. Same as RAM. It's expensive at the higher counts, and over 16GB, you're not likely utilizing it for a game, so adding more isn't going to net a benefit.
Past a certain threshold, the only thing that matters is the video card. And the perks from that are dependant on how it connects to the processor. And that's under the control of the motherboard design and what implementation it uses for PCI Express and what slot you stick it in. Which is back to, are you a motherboard designer?