The MAIN thing to be aware of is to load up on memory. Since these are educational machines the CPU speed, hard drive size and speed, and all the ports and everything else are absolutely fine on a "second from the bottom of the line" type machine. Just add additional memory. Memory makes the biggest performance impact on non-gaming machines because the more memory, the less "swapping" the operating system does to and from disk.
The other thing about Dell is they normally have a "deal" every week for free memory upgrades, free or reduced cost printers, or other stuff. The quality inside the box is excellent. They are not the cheapest, but you open the inside of their boxes and compare -- they use metal where others use plastic, their heat sinks are oversized, the wiring is nicely tabbed down, etc.