Hyper-Threading

Not really that benifical as few programs use Hyper Threading at this time.

Actually Windows XP and Server 2003 take advantage of hyperthreading, and you will find a noticable increase in performance if you enable HT on a machine that supports it (running XP), especially when running several apps at the same time, or when performing CPU intensive jobs (such as rendering).  At any rate I haven't heard anything confirmed about HT adversely affecting performance or stability (there are some extremely unlikely sets of circumstances that have been mentioned in MS KB articles, but nothing a typical person would experience).

I have no idea why Dell would ship machines with this feature initially disabled like they did to my company, especially since they typically ship their machines with XP preloaded.

I have used HT in Windows 2000 Pro and XP Pro. The main advantage of HT is in multitasking. Even when a single program does not take advantage of HT, when you have several tasks running at the same time, you can get about 20-30% performance gain using HT.