RNDP6000-100NAS still worth to upgrade?

At today's used prices, the upgrades are very cheap (under $25US will do it), and well worth it, in IMHO.  A Pro BE model with a Core2 Duo E7500 and 4GB of RAM will out-perform many low-end native OS6 ReadyNAS unless you are using SSDs.  The only things lacking are USB3 and eSATA ports.  The Pro only supports SATA2, but you really don't see much improvement with SATA3 and spinning drives.

You absolutely must update the BIOS in the Pro before you install any processor faster than a Core2 Duo E6600.  I recommend staying away from quad-core processors.  Though some will work, the cooling system just won't keep up.  There is little reason to go to 8GB, and one big on not to -- price.  4GB modules that will work in a Pro are quite expensive because most Windows computers of that era were limited to 4GB and had two slots, so few needed 4GB ones except for servers.  But most servers can use 2Rx4 RAM where the NAS needs 1Rx8.  Most Macs could use more, but could also use 2Rx4.