can the motherboard, harddrive and processor be upgraded and how big can I go for the dell 2400
No, not in the sense that most people would mean.
Yes, but the upgrade won't yield much 'bang for your buck'. Do you have a celeron or a Pentium 4?
2.26 cubic feet?
Here's a thread that's already discussed this and everyone basically told him buy a new PC. The same will apply to your system as well. http://www.techspot.com/vb/topic50736.html
2.4 Ghz celeron