Can I upgrade? If so, what type of CPU?

Not sure if this is the right section, but since my current processor is taking FOREVER to start a game of TF2 and most specifically, the Left 4 Dead 2 demo keeps bugging up with the framerate dropping at random points and the audio looping for minutes into the game (I had to use the "Take a break" thing until it settled down.. yeah. Oh, not only that.. the ridiculous amount of time it takes to change settings and how it kicks me from game afterward.. it's just frustrating as heck.

I'd like to see what processors I can look into for my Dell Optiplex GX620 (I believe it's an ATX midtower design.. at least it uses an ATX standard PSU, the PSU I put in my old HP Pavilion a430n after the stock PSU died works fine in this comp), if any upgrades are open to me.

CPUID Info: http://valid.canardpc.com/show_oc.php?id=810452


Please help.  


EDIT: Also, would I need to take out that GARGANTUAN cooling system on my current CPU and replace it with the one the CPU will come with to keep it cool? Or will the current fan work fine on its own?