for gaming i would recommend more than that aiw card. unless you plan on using the tv tuner and crap you can get another card that is much faster for the same price.
something like this would blow it away in games.
http://www.newegg.com/app/ViewProduc...102-362&depa=1
one problem you might run into is power, this dimensions has a 250 watt poweruspply. it might not be enough for a new card.
If you wannt do the motherboard proc replacement you are better off getting a new system. cases are so cheap these days.
You might also want to check more of your system specs here
http://support.dell.com/support/topi...&~ck=mn&~tab=3
This system originally came with a wide variety of processors that range alot in terms of speed.