It shouldn't affect your frame-rate as it is listed in the game (unless you set V-sync to cap it to that amount), but a 75Hz monitor will only update the image you see 75 times per second. So having your computer render more than double that amount in a game is probably of limited usefulness. Now, with a 144Hz monitor, you would be able to see more of those additional frames, which might help smooth out motion a bit more.
I agree with Aquielisunari that moving up to at least 8GB of RAM could help performance in many games though.