Intel Xeon is a brand of Intel processor which are aimed at servers.
Pentium EE - dual core processors, giving performance similiar to if you physically had two processors in your system.
Pentium D - current generation of Pentium 4 processors.
Athlon 64 - AMD processor, 64-bit.
Opteron - 64-bit processors for servers, basically.
Prescott - name of one of the cores in the Pentium 4 series.
Itanium - Server technology.
Tejas - pinless processor.
Both AMD and Intel now have dual-core technology out; these basically mean performance should in theory double as one core can take the bulk of, say, working on encoding on video while the other core looks after anything else that needs processor power. Basically doesn't tie the processor down as much.
As for what is 64-bit, in a nutshell means it runs a lot faster, twice as fast as the 32-bit processors. RAM upgradablitiy si dependent on the board itself.