Your Mac pro is slower than my Macbook Pro.
6 years in computer years is about double the average life expectancy. You had a good run, let it go.
Or live with Lion/Snow Leopard...
Only based on benchmarks. Crossing over 16 GB of ram makes a real difference. I can do more things at 1 time than you could on your laptop.
6 years may be double the lifetime expectations in the Windows side of the house, but I normally get between 5 - 7 years out of each machine. My G5 is still productive, & the only reason I shut down the G4 tower was because I didn't want to go & buy a new monitor for it.
I can move my drives over to a new system, but I haven't seen anything in either Lion or Mountain Lion that screams "must have".
Cloud integration sounds real good until you realize that you have to go through your ISP to get to the cloud. That doesn't look to be a problem in Europe or Asia, but here in the states, that will be the biggest holdup.