Matt, I would seriously look around for a good, used gaming PC or something before building one from scratch.
Maybe I'm lazy but it only takes screwing something up like frying your motherboard once and you have eaten up most of the money you would save by boing it yourself.
I have added drives to machines, PCI cards, regular stuff and never bothered to build a machine from the ground up. Researching the compatibility of the devices just seems like a pain in the arse.
If you know what you are doing at the technical level AND can locate a sound configuration easily that is within your budget (plus locate all the damn parts) then go for it.
Otherwise look around...
Just my (lazy) .02.
P.S. Doc Jones gave me some really good advice on a machine that I was looking at a few months ago, send him a PM and see if he has a canned config. you can use.