Actually, If you have a PCI-E slot, An average/decent graphic card can cost no more than 50$
You could buy a cheap ATI like this one :
http://cgi.ebay.ca/ATI-Radeon-HD2400PRO-HD-2400-PRO-SCS-256MB-DDR2-Video-/360296280866?pt=PCC_Video_TV_Cards&hash=item53e354f322#ht_2270wt_1137It might not run everything at high detail but it does its job.
If you are looking for performance rather than quality, This card is your best bet.
You should have 1GB or more RAM to run games properly. 2GB is recommended.
I think it supports Pixel and Vertex shader up to 4.0, But I am unsure about this.
I do know it has at least 3.0, As well as DirectX 10 support.
It can run many games, Even Far Cry 2, But not Mafia II.
As for the Pixel Shader emulator...
I must tell you when I used to have a really bad PC, I spent countless days to find such a thing, And I never found anything at all. I say don't waste your time searching for that, As you might just end up with some wasted time.
And last, In my opinion, I think that you will be decieved with MW2. I personally think MW1 is way better than MW2. Death streaks, New perks and killstreaks are just... Lame. But if you think you'll like it, Give it a shot.