You might be surprised though. Half Life 2 takes up less video power and more processor, so you might find that you'll be able to run in High in 1024x768 too.
What i'd do though, is go to 800x600, turn on high, and try turning on some antistropic filtering or a Trilinear one, as that'll help textures look better too.
I liek teh look of 800x600 anyway. It looks sharp, though looks better of course with antialising turned on too.
Have a little play in other words.