Good computer, stuffty GPU.
Even a 4650 is going to have a little bit of trouble running Source games on high detail. The 4200 will be absolutely crap.
nope, my integrated graphics card doesn't lag on blockland. with a dedicated graphics with up to 3gb of video memory i should be good...all i really want this for is minecraft and video/music
Nonono.
GPU VRAM =/= performance.
It plays a factor, but nowhere near as important as how powerful the core is, etc.
e.g The difference between a 6550 and a 6970 (both 1GB models) would be that the 6970 would be approximately 6 times more powerful.
Also, it doesn't have 3GB. That's just what it can share with the system, but if you rely on that you're going to run into problems. The most it will have is 512MB on the chipset itself.