memory upgrading

OWC hace memory

https://eshop.macsales.com/shop/memory/owc/apple-mac-pro/2008

This place has memory that is a little less expensive

https://www.datamemorysystems.com/apple-mac-pro-memory-upgrades/

I used the later for FB DIMMs for a server of the same vintage as the 2008 Mac Pro

If you go to https://www.crucial.com and click Scan Computer it will give you a complete report on exactly what your upgrade option are.

Crucial will certainly scan the Mac Pro but in my experience it has been a long time since Crucial sold RAM for the 2008 model.

I recently bought RAM for my 2008 Mac Pro from Mr Memory UK but only available in 4GB sets ( 2 x 4GB ).

If you have the big silver tower then see:

Mac Pro (Mid 2012 and earlier): How to remove or install memory - Apple Support

The 2008 is near the end. It includes:

"You should install FB-DIMMs in matched pairs of equal size and equal specification. Use the table below and the instructions that follow as guidelines for DIMM placement."

Thus you can use 2 1 GB modules and 2 more if they are same capacity.Make sure you get one made for the Mac Pro since they have the necessary large heatsinks and temperature sensors. There are modules that will fit but those are made for servers and have small heatsinks which are OK for the high air cooling airflows in servers.

This is a Mac Pro 42 lb desktop computer:

this is a later model:

Do you mean a MacBook Pro like this one:

what size screen? what processor speed?