Your CPU is fine, I'm guessing you want a new mobo that is SLI, go with this
. Not saying that the 6600 GT 128 is a bad card it's just you might want to upgrade that to a 7800 GT, What kind of PSU do you have? Tell me the ampage on on it as well. You might have to get a more powerful PSU to handle newer cards. 1 GB of Ram is good (do you have 2 X 512 or 1 GB stick?) to play new games but if you want eye candy you will going to have to have 2 GB.
Do you have to have a new case? Well I know Lian-Li make the best cases but there pretty expensive, you might want to look at the Antec P180 case, but if your on a budget this
is a good case for the money, I got the silver one but I'm sure they are all the same, and it's great as far as I can tell for the price, but there may be better but I'm just recommending it (btw don't get the ones with the power supply in them, Aspire makes great budget cases but make low-quality PSUs like most that come with cases).