I play both CS 1.6 and CSS. I find that unless you have a good video card, the graphics will get choppy in CSS. As for 1.6, you could basically have anything and it will still work.
I have all three versions, but I play Source everyday. It runs very bad on my PC, 30-50 fps on Dust maps with everything on low and minimized. Still loving it though.
I used to play CS 1.6 (I was in a clan and we did cal-o) I now just play CS Source casually, it's alot more fun then 1.6 imo (and it runs pretty good on my comp)