Just to re-itterate....
YOU DONT NEED LEOPARD TO USE SSD OR PLAYBACK HD CONTENT!!!!
Tiger works just fine with both - disable spotlight by putting the drives into 'privacy' - and disable dashboard. Turn off all power-saving etc.
If you already have Tiger - stick with it - dont bother upgrading. Theres no benefit. X1900XT is fine on 2007 Macs
Would recommend 2G Ram (min) for 2007 Macs
If you have Leopard - stick with it - dont downgrade!
Nvidia Ge-force 8800GT is fine on 2008 Macs
Would recommend 4G Ram (min) for 2008 Macs
Re 7500 vs 7000 SSD - theres negligible difference in performance - save your money and go with the 7000 if its available.