Go with a small SSD for OS drive, like an 80GB Intel X25-M. That leaves enough room to run a few games from it too. The difference in something like WOW when loading textures is immense and the whole system is much snappier.
I voted to build from scratch, but would submit that there is a third (5th?) way.
Buy a second hand server from one of the online salvage places.
You'll get something two or less years old, with serious memory buses, big Power supply, high speed HD's and a ton of RAM.
Probably for about half the cost of your other options.
You just need to add a decent video card (check the type of expansion slots)
There are several websites in Australia that resell this sort of stuff that is dumped by corporations when it comes out of warranty, sold for cents on the dollar. They also buy forecliosure items, which should be a fertile furrow in your part of the world. :-(
I voted build your own but if you are not up on all the components it can be a bit of a pain. On the other hand building from the ground up gives you intimate knowledge of your computer.
I predict the poll will tell you to build your own.
I submit that unless you are very specific about your requirements, just get a pre-built box. BYO takes a LOT of fiddly time that could be better spent, well, gaming. :P Also, from my personal experience, the savings on BYO are pretty small when compared to a big-box manufacturer.
-Alaron (who was able to buy two Dells comparably spec'ced to the homebuild I had planned out, a year ago).
I voted build from scratch, but I feel your pain. I used to build all my systems from scratch, but the last one was out of the box from Sony. It takes a lot of time and being that I do not work at a job that keeps me up to speed on all the hardware components any more, scratch building is a lot more difficult.