Cinebench will test out your GPU. If you really want to stress the CPU and GPU together, get the smallLuxGPU application:http://www.macupdate.com/app/mac/33632/smallluxgpu/
Geekbench will run some CPU-only tests. There is a RAM tester here, though not free:http://www.memtestosx.org/joomla/index.php
Your hard drive will probably be the component most likely to suffer a failure but there aren't really any apps that do prolonged reads/writes. You can just use the Finder though and duplicate lots of small files so select a small file, then command-d, command-a, command-d etc until you hit about 30,000-40,000 or so. It'll only take about 15 repeated commands.
To do a stress test sequential write, just erase the free space on the drive using Disk Utility.
If you can post a few of your scores from the benchmark apps, those would be good to know. Run each test 3 times or so. Cinebench has a CPU and GPU test, you don't have to bother with the single core test, just multi-CPU and OpenGL.