I'll chip in on this message. I bought HL2 when it first came out and it stuttered a lot with my 3.0ghz P4 and 6600GT, but ran real smooth otherwise. Upgrading to 1.5gb of DDR2 cured ALL of it with NO other changes!
Fast forward to present. HL2-Epi 2 stutters since installation > same 3.0 P4 but now with a 7900GS! Everything maxed at 1680x1050, full HDR & motion blur, 8xAF/4xAA. Getting rid of 4xAA & motion blur did NOT help much, if at all.
Curiously, running the exact SAME game config & resolution on my now old m170 (below) with 2gb DDR2 runs perfectly - NO stuttering! Of course it's not quite as fast as the 7900GS, but the additional ram gets rid of it. AFAIK the Pentium M760 @ 2.0ghz is a faster gpu for gaming than my old P4, but it's basically a wash.
BTW I run Steam/Orange Box on a removable USB hard drive and can easily swap from PC to laptop. In fact, I now run ALL my games like this. IIRC there were a couple that had some problems, but I was able to fix those. If anyone else does this, just make sure the USB drive has the SAME DRIVE LETTER for all the PCs you're using - those partitioned harddrives on some laptops (drive letter C/D) will balk unless you reassign the letters.