With using a kernel rpm, if you installed it with the rpm -i switch, not the -U switch it "should" leave your old kernel intact and you can select at your grub/lilo boot prompt. You'll need to satisfy any dependencies yourself. Packages may experience problems with the new kernel but probably shouldn't if they are new'ish. Check the documentation for the kernel and the packages and see if they discuss any compatiblilty problems. Have a look around on google, that sort of stuff.
Make sure you backup before doing any sort of kernel work!!!!!!