I played Half Life 1 on a Pentium 133 at 640x480 in software mode, run fine (Win95). It also depends on your memory. I had 40MB, but I remember that my initial 8MB RAM were not enough to play even Age of Empire back in 1997 (horrible mouse lags on bigger mid-sized maps).
Hell, I had to run Quake 2 at 512x384. SiN was only playable at 384x288, and looked awful.