Steve,
Here, at the top, is a short review of C1E and EIST. If your processor does not support EIST you have no chance:
https://www.reddit.com/r/overclocking/comments/39sirq/intel_speed_step_on_or_off/
If you look up the Xeon processors on Wiki you will find:
https://en.wikipedia.org/wiki/List_of_Intel_Xeon_microprocessors
If you then search in that article for the code name of the Xeon group that includes the E5335 (Clovertown) you will find this statement: "EIST support all except E5310, E5335".
I'd recommend running two of the E5345 Xeons for this workstation, and that processor does support EIST. I'd specifically buy only the latest Intel stepping of that (the SLAEJ). You can search eBay for SLAEJ and find a pair now for about 14.00 USD, free shipping.
I've posted quite a bit here about that sSpec code processor for the xw6400, which has the latest Intel improvements built in when compared to the two earlier steppings of the E5345. There are roughly 10% of xw6400 motherboards that cannot run this processor, but I'd think since you have the E5335 running you will be able to run the E5345 SLAEJ version. The E5345 is the fastest Xeon that HP ever supported for the xw6400, and when you can run 1 reliably you can always run 2. I stress-test them with Prime95, and if it will fail you'll know that within about a minute. I'd start with one, and add the second after you prove the first will work. You generally cannot run a processor that is not on the HP approved list because it will not be in the "microcode" in BIOS that controls which processors can boot. There are some rare exceptions.
I like the Noctua thermal paste the best..... have done this conversion on many xw6400s. If I can't run two SLAEJ E5345 (2.33 GHz) in a particular xw6400 then I run two of the fastest dual cores (E5160, SLAG9 sSpec code, 3.0 GHz), and that has worked out quite well.
I agree on always using the latest BIOS version, of course. For others.... If I were buying a xw workstation right now I'd favor a xw6600 running two E5450 quad core processors, sSpec code SLBBM.