The Dell 2000 line typically has the cheapest possible parts. That means several things.
1) Frequently the 2000 line has a slower FSB due to using the most basic chipset. This often degrades performance by 5%-10%.
2) Frequently the 2000 line won't handle the top speed CPUs.
3) Frequently the 2000 line won't have high speed memory - another 5%-10% performance drop.
4) Frequently the 2000 line will have the lowest upgrade capacity (fewer drives, lower max memory, etc).
5) Frequently the 2000 line will have a wimpy power supply (related to #4).
6) Frequently the 2000 line won't have an AGP slot - meaning it comes with slow graphics and you can't install the fastest video cards even if you wanted to.
Basically if you play 3D games, the 2000 line isn't for you. If you are someone who just needs Word and the Internet, the 2000 line is a great buy. If you want to upgrade a lot, don't get the 2000 line. If you are like the normal person and never upgrade then the 2000 line is fine.
If purchased through the home department the 4000 line is often just $50 to $100 more. That can be quite a good investment. If you purchase through the small buisness department, often the 4000 line is the same price after rebate.