What else will this comp be used for? the 5670 will be a little stressed at 1920x1080 but should do pretty well for both apps as neither are GPU hungry.
SC2 will be fine on this system. But the MMORPGs are a different creature when trying to run at max settings. When you're dealing with a populated place, there's TONS of textures that have to be loaded and rendered because every avatar has this little unique trinket or that. There's no way to pre-load all those textures so the graphics card ends up pulling the data directly from the hard drive. This takes time. The best thing you can do for is get a SDD because the random seek/read times are so much better than mechanical drives. If you don't use your computer for a lot, a 30gb might get you by, but a 60 gb should fit in the budget with some tweaks. The main one would be drop the i5 750 and look more towards an Athlon II x3 or above.
If this is something you'd consider, we can start picking parts. You shopping newegg.ca or nxic?
You're new comp will be more than capable of homework and music will really be an issue of your library size if you need to keep the old PC around or not. You could always pull the old HDD and use it as a storage drive in the new one.
Shopping local is always great, because if there's a problem, you have a place to throw it. And saving on shipping is good too.
Newegg AMD A II x3 440 $77. Great price/performance value there! The 925 is nice too, but doesn't really save the kind of money you need if you're trying to get an SSD.
If you're not going to OC and not doing CF, that's a lot more PSU than you need. The earthwatts you have above is enough actually even for the 5770.
That case you have is good enough. I have a similar case that only has the one front fan, one side, and optional for the rear. I OC and my temps are still nice and cool. Cases have a basic job to keep your stuff together. Its a pretty easy job to do. If you like the look of your current case and want to save some cash, keep using it.
I'm sure you see much bigger and better aircraft than I. I work for the airport and don't do any handling of the aircraft myself. But air carrier wise, A320s, MD80s, B717s, DC9 (can't wait for those to go away.) Fedex brings in a pair of A310s. Then the RJs round out the rest for us.
When looking at the 955, be sure to get the model with the BGMBOX in the model. Its the newer stepping, so its been revised a touch over the older ones.
Here's my quick build, there may be better deals yet and cruizing other sites might be worth it just like that SSD you found. Great deal there.