Rndp6000

RAM must be unbuffered, non-ECC, and of 2Rx8 configuration (also sometimes called "low density"), not 2Rx4 (sometimes called "high density" or labeled "AMD Only").  4GB modules of that type are hard to find and expensive because computers that used them typically had 32-bit OSes limited to 4GB of RAM, so there was little market for them.  2GB modules are cheap, especially used on eBay.  Specs for the Ultra6 memory are the same, BTW.

I have had success with OCZ and AData brands low CL versions, and many swear by Patriot.  But they don't really seam to be picky.  The RAM is not dual-channel, so the sticks don't have to be matched.  I've not benchmarked how much improvement I get with the low CL sticks.

As for the CPU, if it's a -100 model NAS, it has a Pentium Dual E2160 @ 1.80GHz.  If it's a -200, it has a Pentium Dual E5300 @ 2.60GHz.  The fastest processor that can be installed with stock cooling is a Core2 Duo E7600 @ 3.06GHz, but then only if you have the latest BIOS (07/26/2010 FLAME6-MB V2.0), and the update must be done before you upgrade the processor.  If you are still running OS4.2.x, then a BIOS update is easy, except on some very early models that cannot be updated to that version (so i've read here -- never saw one).  It's much more difficult if you've upgraded to OS6 -- I found it easier to downgrade back to 4.2, update the bios, and re-convert (all done with a scratch drive, not a real volume).  FYI, the motherboard FSB is stuck at 800MHz, even with a processor like the E7600 that supports 1066MHz.

Frankly, the speed improvement from the E5300 to the E7600 is hardly worth the effort, even though you can buy the E7600 dirt cheap today, in large part because you don't get the FSB boost.  The upgrade from the E2160 is well worth it, IMHO, especially if you are going to run OS6.

Years ago, some tinkered with a quad-core of the same generation.  It worked, but overheated badly, and there is no room for a bigger heat sink or fan.  So unless you want to get into some radical cooling, don't do it.  Just buy a newer NAS if you need that kind of processing power.

The orignal Pro BE/Pro Pioneer (-100's), the Ultra4Plus, and the Pro6 (-200 model) all have the same motheboard (save those few early systems I mentioned earier).  In OS4.2.x, they just identify themselves differently to the OS and it turns on or off certain options.  If you upgrade to OS6, only the processor speed is going to make any difference -- it doesn't care which model you have.

And while you didn't ask, it seems the Ultra6Plus can have either processor.  I understand it was first intended to ship with the slower one when the Pro6 was shipping with the faster one.  But I have had two that had the faster one, so I guess the cost of building two configurations got to be more than the differential cost of the processors.