Build your own is the most cost effective and practical way about it.
2 montiors= Any non-integrated video card
watch tv= TV tuner card, depending on price/quality the above applies here
You can build an econo computer with those parts for less than $400(no monitor obviously). A more media center like computer will probably push you up a few hundred more for a better CPU+motherboard and RAM. An all out gaming computer, near top of the line everything will cost you around $700-1000 depending on your needs(1-2gb video card, lots of ram, HDMI-out, tv capture,etc).
I recommend taking a look at tigerdirect.com and building your own or building off a barebones kit. They have some guides on there, most of it is self explanatory, just requires some reading.
