2.40 Ghz, 3 GB of RAM, 80gb hdd, 80gb secondary hdd, 80mm fan in the back, an 80 mm fan on the side, and 120 mm fan in the front. He had an nvidia geforce 210 in there and was priced at $300, but seeing as how I would take out the video card anyway and upgrade to something better he would drop the price down to $200...My question is without the video card is it worth $200? http://tricities.craigslist.org/sys/1915134960.html
Without knowing motherboard make/model or CPU make/model its hard to say. For a little more than that price you can get a cheap Dell thats probably a better machine than what you are asking about.
If I put an OK video card in there around the $70-80 range would it be able to play, say COD MW2, or Mass Effect 2?
Depends what 2.4Ghz it is, he doesn't say. If it's an old Intel Celeron, you'll struggle to run those games. If it's a Core 2 Duo you'll be fine (though you will definitely need to upgrade the graphics card, an $80 HD5670 would do the job). However, have you considered the fact that judging by the case, this is an amateur build, probably with low quality components, and it's quite probably very old already? There's a reasonable likelihood this system will break within months of you buying it. Depending on the CPU, it probably wouldn't be worth $200 either. Be wary of this idea.