$800-ish new build "gaming" computer

I have never put together a computer before, I have added a new graphics card and a wireless network card and that's about all the tinkering I have done with the inside of my computer. I am familiar with all the parts in a computer, but I'm not sure how to figure out how to pick and choose computer parts, like knowing if the mother board is compatible with the other parts of the computer, or how much power and/or how much cooling is required.

Approximate Purchase Date: I plan on building the computer after September 1st, after I get back from my trip, so that parts aren't waiting outside my door  

Budget Range: (e.g.: 300-400) Around $800 (could be just for the desktop), I would prefer cheaper, but I can go a little bit more if it would make a big upgrade (better bang for the buck) Monitor something cheap, but nice to look at for hours.  

System Usage from Most to Least Important: Most important would be, being able to play video games. I don't play the most highly intensive games, but I do play Diablo 3 and plan on playing Guild Wars 2 and Starcraft Heart of the swarm. Other than that I would just be using the pc for excel, word, ect and internet.

Are you buying a monitor: Yes

Parts to Upgrade: This is my current computer http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00...
The only thing added was GT 8600 Graphics card (I got it a while ago). You will laugh and understand why I need to upgrade.   I don't need to reuse any of those parts, I'm just saying I have them.  

Do you need to buy OS: Yes

Preferred Website(s) for Parts: Any

Location: Waterford, MI, USA

Parts Preferences: No preference

Overclocking: Maybe, I have never done it before

SLI or Crossfire: Maybe, I have never used either

Your Monitor Resolution: I think I would like 1280x1024, but this isn't a huge deal to me, anything you guys suggest I would consider.

Additional Comments: I would like a monitor that is nice to look at, I'm tired of looking at gray cases. It doesn't need lights necessarily.

And Most Importantly, Why Are You Upgrading: Because my computer is about 7 years old and was running Diablo 3 and Starcraft 2 very slowly ha not sure how it even managed it. My monitor recently broke (I think it was due to me moving), so I just figured since I was going to get a new monitor that I could get a new desktop as well. And I saw many people saying that you should build your own and not have it per-built.

I am also looking for a new gaming mouse, my current one likes to double click sometimes, if you guys have any suggestions about a new mouse I would gladly look into it. It doesn't have to be anything fancy, just something that may have a few extra buttons to set key bindings to. Just not Wireless.

I would greatly appreciate any help for a newbie