First of all, thank you all of you
Yes, I don't need a high-end gaming system. I just need something faster - right now my XP desktop (Pentium M purchased in 2005) is too too slow. Took a good 10-15 minute to start up windows (after logon), loading Firefox is slow, and take 4 hours to render my HD video clips to DVD.
I like the Dell XPS8000 because of its case, so cool that there is a space for me to put my iphone while connected. But I really doubted if the 350Watt power is sufficient since my desktop (purchased in 2005) was already running 450Watt.
Regarding building my own - I am not sure if I am really capable. I can install a DVD driver, or even a harddrive. But everytime I have to replace a fan or power supply, I have to go to a computer store. Now we are talking about installing motherboards and connecting those cables. Looks like a mission impossible to me.
I did a quote with a local computer store. It is causing slightly higher than the Dell system (around $200 CAD more ). This includes a labour charge ($50) for assembly. But I am getting the same specs and better components (e.g. 8 GB 1333 DDR3 RAM, Asus P7D5 motherboard, Antec Sonata case with 500Watt power supply, etc).
Back to a very basic question. I don't play games. The computer serves for the following purpose -
50% programming with little web design (Java, Rails, Dreamweaver)
30% web surfing, downloads
10% watch movie (mostly rmvb/avi files, seldom watch a DVD in computer)
10% video editing and rendering (i.e. from my SONY DV to DVD)
I do like turning on my itunes to play music while I work.
What configuration would you suggest? Budget is $1500 CAD - no need to use up the budget, since if money left I can buy a new monitor

I would like to go with Windows 7 Ultimate and maybe i5 or i7 CPU.
I am not really sure if I know the difference (at least if that is useful for me) in the following -
1. Core 2 Quad vs i5 vs i7
2. Dual Channel vs Triple Channel RAM (6GB, 8GB, 9GB)
3. DDR2 vs DDR3
Thanks again!