As a developer and a photographer, I have been struggling for a while with the limitations of a four year old laptop compared to requirements of working with today's technology. I had the Apple max RAM of 4GB installed, and had already upgraded the original 250GB HD to 500GB a couple years ago. Still, I was looking at a new MacBook Pro in order to have enough resources for today's photo processing software and development tools.
Armed with a new 4GB RAM chip and a new 750GB hard drive, I spent most of a weekend breathing new life into my reliable but aging 2008 MacBook Pro.
The RAM upgrade was dead simple. Just pop out one of the 2GB chips and insert the new 4GB chip and you are done. The iFixit guide is clear and makes a basic process even easier. This took only a few minutes. Button things up, power the system on and - bam - 50% more RAM.
The most time consuming part of the hard drive replacement is duplicating the current drive. I mounted the new drive in an external USB enclosure I already had on hand (about $40 from Amazon, if I remember correctly) in order to copy the entire contents of my current drive to the new one. Carbon Copy Cloner is great for this, although there are other tools as well. Be sure to format the new drive with a GUID partition type and copy all the files to make the external drive bootable.
When the drive finishes copying, test it out by rebooting and holding down the Option key to select the external drive as the boot drive. Verify everything is there.
After verifying the new drive contains all your data, iFixit hard drive replacement guide enables anyone that can use a screwdriver to open up the case and swap the hard drives.
After the drive swap and memory upgrade, it was time to upgrade to Lion (OS X 10.7), and then the inevitable chasing down of software build tool dependencies, but that's another story.
The new system I want is always around $3k, which is not exactly pocket change these days. For a few hundred bucks I was able to extend the useful life of the last $3k I spent on my 2008 MacBook Pro. I've still got my eye on a new MacBook Pro, but for now I have "sharpened my saw" enough to efficiently continue cutting through my daily work load.
Don't be intimidated by technology or spend money on a new system if you don't really need one. Use the iFixit guides to get the most out of your current hardware.