I'm getting lag in games when I shouldn't be?

Lately I have been having FPS and lag problems in games, specifically non-demanding games. These games include: GTA San Andreas, No More Room in Hell, Counter Strike Global Offensive, Farcry 2, Minecraft, Skyrim, and a slightly more demanding game; Contagion. I'll include a list below showing specific problems in these games, but first I'll list my computer specifications. BTW: All my drivers are up to date.

Windows 7 64 bit
8gb Ram
Nvidea Geforce GT 640 2gb DDR3
Intel Core 2 Quad Q6600 2.4 ghz

GTA San Andreas - All max settings, I get around 70-120 fps average, but I get mouse lag in specific areas on the map
No More Room in Hell - All max, slight stuttering and sometimes mouse lag
CS: GO - All Medium settings, I get usually a constant 60 fps, but when I put it on high/max settings, my fps drops drastically and I get mouse lag(I pass recommended requirements?)
Farcry2 - All medium/high settings, I get mouse lag and fps issues, usually only about 30-35 fps on average, and mouse lag in MANY places on the map (usually the merc camps and whatnot)
Minecraft - All normal settings, major stuttering issues when sprinting, fps issues, I usually get about 50-60 fps on average but it does drop often (mostly when sprinting)
Skyrim - All medium/high settings, about 30-40 fps average, some mouse lag, some stuttering
Contagion - All medium/high settings, low fps, stuttering

I'm wondering why I can't max out all these games, and is there a way to fix the lag issues?

Added (1). Is there a way to fix the lag issues without lowering my settings, is what I mean lol. I mean, I don't care TOO much about how the game looks, as long as it looks good I'll be happy haha. I know that the graphics don't make a game good, the gameplay does. But I pass the recommended requirements for all these games, I should be able to run them on all max settings with a 1440x900 resolution, right?

Perhaps you've disabled the GT 640 as your primary GPU? Click your Home button, then search for 'dxdiag', once it's opened, open the Display tab on the top left and make sure your 640 is listed there.

Try updating your graphics card drivers or reinstall windows or reinstall games
also for minecraft
the sprint stuttering is normal
its the infamous sprint glitch, everyone gets it

The review of that video card doesn't look good.

San Andreas Video Graphics PC Specs:
Recommended: 128(+) MB Video Card with DirectX 9 compatible drivers ("GeForce4" or better)
It can get by with the Minimum:
64MB Video Card with DirectX 9 compatible drivers ("GeForce3" or better)