Can I replace 2 registered 8 GB Modules instead of 2 existent 2 GB Modules (MacPro Mid 2010 2 x 4 cores)

Of course, provided they're listed as compatible with the Mac Pro. Make sure the memory speed is appropriate to your model.

Matt

Edit: if all the current RAM is registered, you'll be OK. Otherwise, you will have problems:

http://support.apple.com/kb/HT4584

Matt

Some 8GB DIMMs were known to not work with others, that has changed, but like Registered and Buffered, beware before you buy.

You might want to just jump to 2x8GB on each bank even.

Using 4th slot is also not a good idea so aim for 6 DIMMs.

Buy from a Mac-centric dealer whose policy is:

"It works in your Mac,

Or your money Back."

This is such a dealer who sells at very resonable prices, and in my experience has offered good Mac support:

http://www.datamemorysystems.com/apple-mac-pro-8-core-2-x-quad-core-intel-xeon-w estmere-2-4ghz-mc561ll/a-mid-2010-memory-upgrades/

Prices at this writing:

with 8G DIMM at US$79,

and 4G DIMM at US$53.25

nothing smaller than an 8GB DIMM makes sense when buying new.

EDIT: Macsales.com also offers these, as well as a complete 3@16GB kit. But they are charging a bit more for theirs.