Hello Hankey and others...
I would just like to say that I've played HL2 on my P4-2.4,1Gb ram,radeon 9700pro@agp8x,and winxp sp2, and there we no problems at all.
So...Your configuration shouldnt be problematic here...and I also think changing agp from 8x to 4x wouldnt make any difference (btw...do You run ATI's Radeon? With heatsinks on ram? or some of the first samples without passive ram coolers on it? - because only in this case there could be problems with overheating) But if You'd like to change to 4x anyway...just go to control panel,click display, then settings,advanced, and SmartGart...then change to 4x...and click apply...(maybe You would need to restart...I am not sure about that)...and thats it...
Anyway...I did a little research...and found that some people said that there is some problem in the original HL2 and that Valve admitted there is an error and that patch will be released soon(post dated 24.12.2004)...also that game crashes at random after 2-20 minutes of playing it...and that it also doesnt happen to everybody (didnt happen to me - so I am living proof that this is real

)...also...that it could happen if You have OCed machine (but I guess You havent overclocked it yet

)...and so on....
So...there are many many posts in that particular forum...so I believe You could find even more useful informations there...just click
HERE and check it out....
Well...that would be all for now...let us know later if You managed to fix the problem...or....shall I do a Complete-Expert-Ultra-Superb-Research about this...

Enjoy!
p.s.Just remembered...at what details do You run the game?...in case You are running with some ultra details

....try lowering everything to default...
Byeeeeeeeeee