Yeah you're not going to get very far trying to play X3 on anything less than a supercomputer, it's pretty good but terribly optimized and will melt your computer. Don't know about X1 or X2 though.

In that case I reiterate the I-War games.

Independence War Deluxe - GOG.com
Independence War 2: Edge of Chaos - GOG.com

First one's more linear and tight, but they hadn't quite refined everything yet, whereas the second one does better in the general gameplay regard. They're old now so your computer won't be a problem!