How and why to upgrade memory and cpu

The Pro 6 has a Pentium Dual-Core E5300  @ 2.60GHz.  You don't get much of a boost going to a Core2 Duo E7500 @ 2.93G, which is the fastest it will accept unless you go with a quad-core, which requires special attention to cooling since it has higher power.

The old Pro BE and Pioneer and the Ultra6Plus use the Pentium Dual-Core  E2160  @ 1.80GHz, which does get quite a boost with the Core2 Duo.  (Though my Ultra6Plus came with the E5300, so those looking to upgrade one of them should check dmesg.log to see what you have).

My opinion is leave the processor alone on the newer Pro6 models and just upgrade the memory.

As for memory, it comes with a 1GB stick.  You can cheaply upgrade to two 2GB sticks of PC6400 DDR2.  It will accept some 4MB sticks (2Rx8, aka low density, non-ECC).  They are less common and were a lot more expensive, but the price is dropping as the technology gets older.  4GB is typically enough, anyway.  I've used OCZ and GSkill low latency (5-4-4) and have had no issues.  Many swear by Crucial, and the standard RAM is typically Hynix.  I've found them to be much less picky than once rumored in this forum.