Either look pretty good, you're pretty much safe with a 512 MB card, with DX10 support, since it should be very backwards compatible with DX9 in the sense of less lag, more FPS.
EDIT:
Since Counter-Strike Source or CS, whatever you play (I'm assuming CSS because I wouldn't believe you'd get around 30 FPS in original 1.6).. but since you play CSS that's DX9, and not many games have been written with DX10, from what I've seen....