Dell's are quiet, but there is a lot more to this than meets the eye. Dell's are quite proprietary:
1) they use motherboards that are made just for them
2) some are split so that they work in those "clamshell" case can open, they sometimes have just 1 or 2 PCI slots open
3) they use "closed" BIOS's -- virtually nothing can be tweaked
4) their power supplies are *not* the same as standard ATX and if you use a standard ATX PS, you may just destroy the MB.
5) they preload all the OS and software onto a hidden partition on the HD and this keeps you from ever using all of the capacity, and you can never reformat -- you will loose your copy of Windows if you did!
6) Rambus RDRAM is more expensive, and you might not have any open slots
The choices you are presenting are almost polar opposites: one is predigested and the other DIY. Also, I'm partial to Athlons.
For your extra $100 you get double the HD and RAM, and you have the ability to expand in a Sonata. And if you want to save some money, consider the Evercase 4252 with a Fortron P300XFPN 300watt PS, and possibly a Tbred 'B' 2100+. The systems in my signature cost me ~$525 each and with a 60GB instead of the 120GB that I have, they would be ~$425!
I would have liked to upgrade to the A7N8X Deluxe, which would have bumped it up to $500 again. Also, you could save some on the RAM if you got a 266mHZ bus Athlon and don't plan on overclocking, but you should buy a 333mHz FSB model if you can afford it.