Swapping a hard disk is very easy. There are two ways to do this.
(Long Way) buy a new hard disk and install osx and all your apps again.
(Short Way) buy a new hard disk and a 2.5 sata caddy and use your osx disk to clone the exisiting disk. this way it will retain the apps and settings. However if your current hard disk is failing and cuasing issues the first way might be the best option.
One thing to consider. Not sure of where you are located but in the UK the price off Solid State Disks are plumiting. You can get a 128 GB disk for around £70-£90 and a 60GB for £45-£60. SSD's are 10x faster than conventional disks. However you lack the large space sizes of the disks. For example you can get a 500GB hard disk for £40-£60.
guides for replacing hard disks are on
http://www.ifixit.com/
Mike