HELP - Mac Pro Running Very Slow (After Additional RAM Installed)

Question

I have recently upgraded my Mac Pro (2007) and increased the original memory up to 10 GB 667 MHz DDR2-F8-DIMM.
I also added an optical disk drive as the original would not play some of the newer DVDs/DVRs. Believe me, I'm not much of a do-it-your-selfer, but was encouraged by How-To DVDs on the net on how to install memory and the optical drive.
I am using Mac OS 10.5.8 and the original processor is 2 x 3 GHz Dual-Core Intel Xeon.
Just when I thought all was well, over the last few weeks my MAC has been getting slower and slower. Now, whenever I re-start or shut down, the Mac can take upwards of 5 minutes to come up. It hangs on the blue screen for what seems to be an eternity. Now whenever I use Safari or Firefox, the rainbow wheel spins and spins and this did not happen much before my "upgrade".
If someone would be kind enough to walk me through or educate me on how/why this is happening, I would be very grateful!
Sincerely,
Don

Answer 1

Did you try with just the new RAM? what was it, and you added.... 4 x2GB?
Riser A: 2-2-512-512
Riser B: 2-2-512-512
The ideal balance is symmetrical sets of four.
Clone your system and run Disk Warrior, reformat your fastest drive for system, and restore with SuperDuper. Or try starting over with clean install, would be useful as an emergency and backup, especially as it seems possibly overdue for some maintenance.
Make sure you have 30-40% free on disk drive, and add more if you haven't to separate scratch and data from system.
Run AHT and something like Memtest or Rember. Memtest you can run 40 concurrently each in its own Terminal window makes it go fast for even 5 cycles.