Is Gigabyte GTX 550ti 1024MB a good Graphics card?

32bit Operating systems only recognize up to 3gigs max of RAM, you said you have 4 gigs installed so you are effectively not using 1 full gig of ram that is just in limbo and unusable. Not the best for gaming. Processer and everything else looks good. I personally don't like win 8. If you are able you want to upgrade to the full 64bit version of Win 8 or roll back to 64 bit Win7 to get full use out of your RAM that will make a huge difference in your gaming ability

Since its clocked at a low 2.4ghz, i'm assuming its a core 2 quad.

its a decent lower mid range graphics card. It should play any game easily at 1920x1080 resolution, but on more modern or demanding games it will likely be on low-med settings.

Logan is correct, 32 bit is only supposed to recognize 2 gigs but Windows recognize 3 gigs. Upgrading to 64 bit windows will not give a significant performance boost.
For minecraft, your graphics card is overkill (minecraft is cpu intensive not gpu intensive). It will easily handle Black ops 2.
I can easily get 80 fps on minecraft with a gts250