The fact is: New machine equals to money for new case, hard disks, new dvd readers and new ports (usb, audio, video, etc..), that even if single the have little cost all together it's markable.
A 150$ ~, while, as I can understand, the cost of the motherboard+ram+cpu+graphic is around 350$-400$ for mid settings.
So my aim was to keep this "common" things unnecessary to change and, in matter of facts, make a new machine mantaining them.
See: In Italy, a 4gb ram is 30~ euros. My father paid 30 euros to let someone put a 2gb ram in his comp..
Well, aside from replacing the case, hdd, and peripherals/monitor, you're basically building a new computer. The Motherboard, CPU, GPU, RAM, and (potentially) PSU are all going to need replacement. Sorry if I was a bit unclear about that. It'll definitely work that way, just requires a bit of disassembly work. Make sure that the new motherboard and GPU will fit the dimensions of your case.
For about $400 (USD), you're going to have to stick to AMD. I don't think you'll be able to hit mid settings comfortably though, probably going to be more comfortable at low/mid, particularly because of how much the game stresses the CPU. I'd start by looking at the Phenom II x4 955
and probably the ATI HD 5770 if you can find a good deal on it somewhere. Then you'd just need an AM3 socket motherboard (ASUS, Gigabyte, MSI are all fine choices) and a pair of DDR3 RAM sticks. Check the wattage rating of your current PSU, too. I'm imagining it's at least 4 years old? If you don't have the money to replace it now but it's still like, 550 watts or something, you can wait on that I suppose.