Your computer CAN go to 10.7, but no further, according to this document:
OS X Lion system requirements
To use Lion, make sure your computer has the following:
- An Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- Mac OS X v10.6.6 or later to install via the Mac App Store (v10.6.8 recommended); you can install without Mac OS X v10.6 by using an OS X Lion USB Thumb Drive [no longer available]
- 7 GB of available disk space
- 2 GB of RAM
If your Mac does not meet these requirements, you will need to upgrade your Mac before installing Lion.
To obtain 10.7 Lion, you will need to CALL the Apple Online store and purchase a download code they will email you in a day or two.
It was not instant, the process for some reason is totally manual and takes a day at least to get a key to allow downloading Lion.
All MAC PRO models can be upgraded to 10.7.5, but Apple Store makes us jump through hoops.
FBDIMMs are cheap now and better.
You can add some SSDs, one for system in stock drive bays and two more on Sonnet Tempo or Pro SSD card to host your L4/5 catalogue which does help. Samsung 840 EVO look to be newest and nice option (you don't need to buy 840 Pro series)
You should hve or want to get ATI 5770 from Apple probably.
Check here: http://www.macperformanceguide.com check the Articles link for advice and tips