I'd say any PII from 300Mhz up (on a good BX motherboard) will outpace a K6-2 series CPU.
The truth is that the different motherboards will have an effect on your end perfomance.
Tzen, does the Aptiva that u want to upgrade have the TX chipset? Are you planning on using a K6-2 which will then re-map the 2x multiplier to 6x? (6 x 66Mhz = 400Mhz)
If so - that combination should provide a reasonably decent performing machine - but don't expect it to be like greased lightning
If you are getting stutters in Half Life thats (probably) due to having insufficient RAM - when I first started to play HL I was running a K6-2 300 @ 336Mhz with 64MB of RAM on an Epox MVP3-E. It was also suffering from the stutters too occasionally. I added and extra 64MB of RAM and it was fine

That may not be an option for you if you are running a TX chipset as I'm sure 64MB is the max that it can use??? The K6-2 played HL very nicely - but the game was smoother on my C300/450 setup