[quote name='pacemakerguy']i have Dell
Microsoft Windows XP. Dell Dimension DIMC521
says i have 960 MB of RAM.
how i get more!!![/quote]
Okay I think I see a few problems here. My girlfriend has the same computer, and it is nowhere near up to snuff to run newer games, such as L4D. You might have a dedicated video card, but you probably have the model with the integrated video on the main board. Even if you could get the right parts to run it (memory, new video card), your PSU is only 280 watts.
Your best bet is to save up your pennies and get a reasonably priced gaming PC rather than dump money into this one.