Oh brother! CS graphics push the limits of HL's graphics engine...which is slaughtered by games younger than 8 years. Even a stripped down UT2k3 swamps HL for graphics!
Anyway...
ATI's XT cards tend to have faster G-RAM installed. Usually the Pro cards have generic DDR SDRAM, while the XT cards have the ultra-fast stuff, which is more expensive, hence the 30-100 dollar price hike.