It's XFree86. If there is no newer RPM from Red Hat, I think it's best to stick with what you have. Installing newer version manually can get you in real trouble when you later try upgrading with the Red Hat software again, unless you really know what you are doing.
The most recent stable version of XFree86 is currently 4.3.0. To check which version you have run:
XFree86 usually resides in /usr/X11R6/bin/XFree86 (at least on my Slackware box).