help me,which'll be the best to use in rhel5 ,like yahoo/gtalk in windows

probably pidgin, but you probably shouldn't be using rhel for the sort of system you'd be doing online chat it, that'd be where ubuntu, fedora and co come into the equation, and the installation with those will be much easier.

I would agree with pidgin.

Starting with RHEL5 I think we are seeing a change in RH's product. RHEL5 was released with FF1.5(?, an older FF at any rate) but by 5.2 they switched to FF3. Changing the major revision of an app within a series (say 5.X) is a huge step for RH. I believe that this may be a signal that RH has realized the Linux's rate of change has significantly slowed. You can also see this in their plans to push back RHEL6 an additional year(last I heard). With Fedora's support life only being 13 months(now) combined the improvements being more evolutionary in nature(as opposed to revolutionary in prior years) I have been migrating more systems to Centos (free version of RHEL). This avoids the needless (relatively) churn of doing a fresh install every year. Centos/RHEL has a 5 (?) year support life.

Check the Centos website for lists of alternate repos (RHEL compatible as well). Atrpms comes to mind. Unlike using them(atrpms) with Fedora, they seem to be well behaved with Centos/RHEL.