The quad core will allow more multi-tasking because their are 4 cores to do operations on. I'm not sure I agree with Redmo0n on getting a Q6600 over a E2180, but it depends on how much multi-tasking you really want to accomplish. You're obviously saving a lot of money bo going with the latter.
1GB of RAM is not sufficient these days to do much of the higher end stuff, so you'll want 2GB. If you get 4GB and want to use all of it, you'll have to look into having a 64-bit operating system.
I agree with the hard drive choice by Redmo0n - it's a newer and better hard drive than the one you picked out. But I don't know enough about it to say specifically why it's better.
For the case, just make sure that it has enough ventilation so that things don't burn up if you're doing intense computations/graphics stuff.
Oh, and as for using Newegg vs. TigerDirect, you can look at the E2180 on Newegg:
Newegg.com - Intel Pentium E2180 Allendale 2.0GHz 1MB L2 Cache LGA 775 65W Dual-Core Processor - Processors
It's way cheaper.