Its always significantly cheaper to build it yourself when it comes to computers, unless you break something. Having built dozens of computers in the last decade or so, I've never broken a component, YMMV tho =0. I remember my very first build I didn't know about thermal paste and the default (out of the box) position for the mobo jumpers was to reset the CMOS, and I fucked up the spacers. So at first the comp wouldn't boot, im like wtf, rma'd it rather than take it out and try it on a piece of cardboard to see if the mobo was bad. they sent it back saying it tested fine...I forget how i figured out the CMOS jumper; finally got it booting and it was overheating and resetting like crazy. I forget who explained thermal paste, but that was a major doh. Oh also, my cpu fan was poorly manufactured, and was rubbing the casing, so i had to file that down =p. That computer was an experience=p. But after all that, it had a nice long life, finally something went wrong with the AGP slot and i got psychedelic colors in 3d games randomly, so I retired it =p. Had some fun experiences with matching ram too, lol. But honestly never broken a component, and the only 'additional' cost was the shipping for the manufacturer to test the board. Anyway, the point of the wall of shame was to say you can't possible the as ignorant about builds as I was, and my first build still worked out, yay! On the other hand, one of the out of the box computers i bought had massive heat problems that I believe ended up causing heat damage to every component that eventually caused them to fail. (Except, perhaps, the hard drive... Also would cut off when it got too hot, particularly in 3d games. I believe that build was using one of the first Nvidia 6800 GT'. But again, that computer/those components lasted for a year or 2 before i had any serious failures. )

Fun times. And before everyone makes fun of me for the first computer ignorance, it was a loooong time ago ;p

wrt components, depending on the type of ram you have, you might have to buy new ram as well. Something you can do, if your replacing most of the components, is just buy another case and keep the old one for spillover. (Run a few dps on it.) Win 7 is your friend, also I love my SSD but they are a bit pricey. Also, I couldn't live and function without multiple monitors =p. A intel i7 is also probably the best bet wrt performance.