Dimension 8300 Speedup (no joke)

Situation:  Purchased 8300 around Dec 2004. Advertised as having a CPU with hyperthreading. Around early 2007 I discovered and attempted to activate hyperthreading.  Well, lo and behold, the CPU was the wrong step and apparently was one of the last mfr'd without hyperthreading. (Speccy told me what CPU I had in the motherboard.) I went back to Dell who sent me to the vendor who said that was too long ago and out of warranty etc etc.  What a burn.

My question is: will hyperthreading provide any noticeable speedup in XP compatible 32 bit programs such as some math intensive things like simulators (eg circuit & logic design), grpahic processing, and Office 2003 level spreadsheet / db software (windows hogs)?  If so, any recommendation on obtaining a compatible processor having hyperthreading capability?

Unrelated question: The online diagnostic application via Dell won't install.  Is there an offline version I can install and run?  (Yes all anti virus had been deactivated first) 

Except for the cmos battery, I've never yet had to replace anything on this old computer although I had to buy and then run SpinRite for about 8 hours to correct a non-booting HD issue a couple days ago - panic (No MBR...really?)  Why bother? Because I have 100's of hours of home written applications (VB, some assy, Lisp etc) and 100's of Autocad dwgs and 2000 vintage applications on the HD and it's a real pain to try to get this stuff to run on newer operating systems. 

While Dell customer service may not seem to be very good at providing service, I can't say the same for the system designers. They did good. This old beast is still going strong.

Service Tag removed per privacy policy by Philip Yip. System configuration below...