Server vs Desktop Components

I will be needing a server for scientific processing and database access over the internet and was looking online and it seems servers are very expensive, so what is the difference from desktop builds?

If I get a system much like in my sig, would it work well as a server?

If I change the configuration to be more server-like:

Xeon 3060 instead of E6600
ECC Memory instead of Non-ECC
PCI-E Raid controller instead of built-into the mobo
Any junky vid card

how will this help me? Should I be looking at desktops to use as a server since I have no need for rackmount? Is ECC memory necessary (in what cases does its error-correction come in handy)? Are external raid cards far superior to motherboard ones?

Thanks,
Jo