I love duallies as well
When I decided a month or so ago that I was going to replace my aging Tbird 1100 rig. I costed a single Xp1800+ system and a dual XP1600+ system. The dual was going to be less than £150 extra, the only extra cost being an extra cpu and a server mobo.
I got dual XP1600+s and a K7D Master-L. It rocks.
I didnt notice at first any difference, except a few things. Mulitasking is a lot easier now. I can minimize apps and run others without a noticable slow down. I can play music, burn CDs, run folding at home, read email, browse the internet and even run 3dmark2001SE all simultaneously. Try that on a single CPU system!
I also chose a dual because I am involved with
[email protected], so dual CPUs allow me to do twice the folding with one PC.
The disadvantages? Very few. Inital cost is higher, SMP capable OS is needed like Win2k/XP or Linux, and the damn thing gets hot! But I wouldnt go back. Also if I want to upgrade, I need registered DDR to use more than one slot. I have 512MB in one slot just now, unregistered. It is unlikely I will add more than another 512MB, so I am OK there. A CPU upgrade would need me to buy two CPUs. As much as I would love to upgrade to dual 2400+s when they become available, I just aint got the cash

.
However, dual XP1600+s, 512MB RAM, GeForce 4 Ti4200 Vcard and a 40Gig hard drive should last me a while. And the option is still there to upgrade the RAM, or get dual Xp2800+s, if my wallet ever stretches that far.
If you can afford the cost, and get yourself an SMP aware OS, I doubt you will be disappointed.
David