It depends so many factors other than just the nForce chipset itself. My opinion is that the nForce is just too new in order to make a judgement call for someone else. I personally would like to use an nForce board in my own next system, but not until I hear early reports back on them.
As for performance, early tests show the KT266A boards outperforming the nForce boards pretty much across the board by only a couple percent. I wouldn't take this too seriously though, as the nForce boards are only on their first official driver and bios release. Performance will undoubtedly improve with time. The KT266a has been around a few months and it's based heavily on the kt266 which has proven to be one of the most stable chipsets VIA has done. VIA also releases new 4 in 1 drivers at a RAPID pace to resolve any issues that come up.
These are the issues I'm going over in my mind: Since this is the first bios release and the first official driver release there will more than likely be some bugs. With Nvidia's history with drivers there will likely be MANY versions that contantly improve performance and stability, so in the long term I'd probably say yes, the board will be stable. On the other hand I think it's important to mention that Nvidia is BRAND NEW to the chipset market and don't have a proven track record. I can't really say how stable it will be initially. If it were VIA releasing a new board I could say for certain that it would probably be useable with some bugs.
Early reviews have been mostly postive in the stability department, and it's probably important to note that the nForce chipset is nearly identical to the one in the new XBOX game console, which better be stable or Bill will be mad.
This would indicate to me that the chipset itself has probably been extensively tested for stability as the XBox has been in testing for almost 2 years now. As an engineer when I look at the pictures of the nForce based MBs posted online, the shear number of coils and caps indicates to me that there may have been some timing and noise issues that were very time consuming to resolve.
I hope I've helped you, but more than likely I've probably confused you even more.
If it were me I'd bite the bullet and play the beta tester game, but expect some early issues to be present.
I would NEVER buy a name brand system though, I'd do it myself or have a local store do it for me. Every name brand company cuts corners and quality has really gone down the tube in the past few years at some of the major manufacturers.