Well, what do you want to do with it? For daily usage an upgrade would work wonders. But if you're expecting to do hardcore HD video work or 3d work, then look elsewhere.
The system takes 1.5gb of RAM max PC100/PC133, in 3 sticks of 512mb.
The processor can be upgraded, and there are many options. I'd recommend something with L3 cache.
Not too many options with video cards. If you're not using an ADC monitor (and you shouldn't be if you want to extend the life of your cube!), then i'd recommend flashing a PC Geforce 6200 card. Its fast and cool and fits perfect in the cube, and supports SM 3.0 !
Don't forget to upgrade the hard drive too!