|"754 is the same as socket a, just don't confuse it with the long obsolete slot a"|
WRONG! Socket 754 is socket 754 & is used for Athlon 64's & the Athlon 64 based Sempron.
Socket A, which is also known as socket 462, is used for the older Athlon, Athlon XP, Duron & Athlon XP based Sempron.
"I have seen an AMD 64 4000 on ebay saying its the fastest socket 754. Its core speed is 2.6 Gightz"
That would most likely be a laptop CPU. The fastest desktop S754 is the A64 3700+. Starmicro still carries several different S754 CPUs & Geeks.com has the A64 3200+ (2.2GHz), which is based on the Venice core, for just $13.50:
You can get an additional 10% off at Geeks by using the coupon code: MCEXCLUSIVES
What is it that you're trying to accomplish? Have you considered overclocking what you have? You *should* be able to get the Sempron to run at 2.25GHz with just a few BIOS tweaks. Do you have DDR400 RAM?