I run virtual machines on my computer. I'm a PC guy and I prefer vmware, but there are free virtual machine packages for PC as well. I'd bet there are some free ones also for Mac.
The idea is that the virtual machine software sets up a "fake computer" within your computer. You let it create a large file, and it lies to everything running inside it that it's really it's own hard drive. The software running inside doesn't know the difference, and thinks it has complete control over it's own machine. The virtual machine starts out as a blank hard drive, so you install your O/S, and all the software and files on it. If you've ever remote connected to a computer, it's basically just like that - except it's actually running on your machine. Requires enough technical experience to be able to install an O/S, etc, but that's really about it.
Not saying to not upgrade! Just saying this is an option.