It's more CPU dependent than GPU dependent if I remember correctly.
you are correct, It also taxes the ram a bit if i remember correctly, I know before i upgraded to 8 gigs it would stutter if i used more then 10 or 15 TNT at once but was fine after the upgrade till i hit about 100+ tnt but it was a CPU problem at that point.
I remember someone with a GTX 670 or something and maxed out getting around 80 ish FPS I think. This was VERY long ago. May have changed now but the point is, it doesn`t need a good GPU to play it. Just CPU and RAM.