You don't need more than one SSD.
You can buy WD 10K VelocityRaptors. $100 250GB / $125 500GB / 1TB ?
Or WD Black 1-2TB for data.
Memory is importantHow To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $35
Yes you should replace and upgrade disk drives more like every two years. Yours (and mine) are 6 .5 yrs old now. And huge changes in capacity and performance.
To use an SSD it depends where you put it. There are 2 SATA ports unused on the logicboard you can access. An IcyDock for older Mac Pro only if you use the drive bays. I run the 2 ports out the back.
An ATI 5770 is a great upgrade for Lion.http://www.macupdate.com/app/mac/7032/carbon-copy-cloner
Create an OS X Lion Install disc
How to clone your system:
I currently have three SATA III drives in use in the drive bays. I wish to remove them and add two SSDs. Would they just lock right in the drive bays with the icy dock? As for RAM, I upgrade to 12GB. Is that enough? It seems pretty fast, just when calling the data, there is some lag. The Mac Pro from 2006 seems to run faster/smoother than my new MBP i7. I am thinking because of the 4 GB in additional memory and the 7200 RPM drives vs the MBP's 5400 RPM drive.
I checked on Amazon, and the 10K VelocityRaptors, are SATA drives, not SSDs.