Lol you post this is a section where nobody knows jack about computers (which is unfortunate).
The reason you can't play L4D(2) is because the GeForce 9100 is an onboard graphic solution (a shitty one at that).
The rest of your computer is okay, the AMD Propus being a very good low-budget processor.
Your answer will be below this line, however I'm curious, why/how do you have 6gb of ram on a dual channel motherboard? You should stick to multiples of 2^x (2gb/4gb/8gb).
Anyway;
To answer your question, you're looking at about ~150$ to upgrade your computer.
1) new power supply
2) new video card
Power supply I suggest anywhere from 500w+
SIDE NOTE: If you want a very nice video card/power supply at a very good price (atm) ~200$
Get this: http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.329244
Very nice video card, top tier brand and it supports DX11/Eyefinity.Power supply: (Brands: OCZ, Corsair, Antec); (450w-600w)
Video card: Depends on budget
~~ $100 total
ATI HD4670
~~$150 total
ATI HD4850/HD4870/HD5750
~~$200 total
The combo I linked you.
This is assuming you live in USA, if not then ignore all the prices.
If you need help, lmk.
thanks.