I am using RHEL 6.4. Problem is it takes too much time to boots up? Can you say, what can I do to be out of this situation?
1) Biggest thing is to disable any unnecessary services. mdmonitor, pcscd, isdn iscsi hplip and hidd are pretty safe to disable to start at boot for the average workstation. Unless you're exporting nfs (this doesn't affect mount exports from others), you can disable any service beginning in "rpc" and any portmap/rpcbind services. "gpm" is pretty stupid and can be disabled. If you configure your interfaces statically and don't do anything with vpn or wireless, you can disable NetworkManager. If there are some services that you want to start at boot but aren't dependencies for any other services, you can kick them off in rc.local as backgrounded tasks.
2) If you can afford it, booting from SSD and moving root to that is also going to help out a lot.
3) You can set HDD schedulers via udev and they will take effect earlier in the boot process for the startup to take advantage. It's not going to be enormously beneficial for something like system initialization, but it will help.
4) Edit your GNOME Startup applications to just the stuff you need.
5) When RHEL7 comes out it will have systemd, so consider upgrading the desktops' OS when it gets released later this year.
That's all I can think of. What issues are being caused by the boot time?
First determine how much time booting does take now. For that set an arbitrary but clear point after which you consider the machine booted, and measure that. I recommend going for CPU going down to the background noise level and no more (startup) disc activity. Based on that you have something real to make comparisons instead of some vague feeling of boredom during the bootprocess that needs to be decreased.
Then set yourself a goal on what would be an acceptable boot time.
After that start measuring. Use bootchart. Start throwing things away that are in the kernel that you don't need and build what you need in the kernel (and not in seperate modules).
I stopped myself at booting in 15 seconds (that was back in 2009), an effort that got inspired by a presentation of someone who got booting shaved down to under a second. I did not want to spent more time optimising myself, but there was room for improvement.
Nowadays I spent less time in booting by only doing so every 6-8 weeks. That cuts down the time on booting compared to restarting every day down to about 2%. And if I have to reboot (e.g. for a kernel upgrade) I do so before I go to sleep.