|"I want to purchase IBM 8700 25u from makeet..."|
It appears you're asking about a Lenovo ThinkCentre A60? The 8700-25U uses an AMD CPU not Intel, therefore a Core 2 Duo is NOT supported.
The one in the above link has an older socket AM2 board, single core CPU, only 512MB RAM & weak GeForce 6100 integrated graphics. It's about 3 yrs behind the cutting edge of technology. If decent gaming is what you're after, you'd have to upgrade the CPU, add more RAM, a discrete video card & probably upgrade the power supply. The upgrades alone would cost close to $300 US.
"And if one system has dual core 2.2Ghz X2 + DDR1 + 256MB VGA"
If it uses DDR1 RAM, it's even older than the above system.
"one has single simple processor of 3.5 GHz + DDR2 + 128 MB VGA...."
3.5GHz? That speed doesn't ring a bell. 3.40Ghz & 3.6GHz, but not 3.5GHz.
And when you state VGA, does that mean onboard video (aka integrated graphics) or an actual video card (aka discrete graphics)? The difference between integrated & discrete is HUGE. For the most part, integrated graphics suck.
I think you need to get more detailed info about those systems or you *may* end up with an outdated, poor performing piece of crap.