As a 2008 3.2Ghz 8 core owner I've more or less maxed out my machine with the following:
16GB Ram (yes you can go higher, but Ram for the 2008 models is insanely expensive compared to the newer MacPro's)
Apple 5870 graphics card. (Went with Apple to avoid any pitfalls with a cheaper flashed card). You can go with a higher spec flashed PC card but they might not be 100% compatible.
SSD SSD SSD SSD SSD SSD This one thing breathed so much life into my machine. Picked up a 256GB Crucial M4. Did the bracket hack so it's mounted in the normal internal bays. (super super easy, see below) OS and critical things on the SSD, media and everything else on other giant normal HD's. You can get a SATA III PCIe card, but from my minimal research you can't boot from the SSD when attached. So my SSD maxes out the SATAII connection, but the speed is still damn good compared to rotating disks, and the instant access makes the machine just seem so snappy.
Quick overview on the SSD bracket hack. The Crucial M4's come with a little tray to make the 2.5" SSD become your standard 3.5" size. Of course this isn't compatible with the Apple Drive sleds from the Mac Pro. I forget the Apple drive numbering scheme, but for this example lets say the drive closest to the front is #1, and the back is #4. You will need a normal 3.5" drive in Bay #3 for this to work.
So for a single SSD (my case) you remove the #4 (back) Apple Drive sled, and the normal drive in #3. Then screw the drive tray that came with the M4 into the drive mounted next to it using the side holes on the drive not used by the Apple bracket. So now you have a drive with one "Wing" on the right. The tray just hangs there in space, lined up with the bottom of the drive. Put it back into the MacPro. Then slide the SSD over the tray that's now hovering over Bay #4 and click it into the Mac Pro's backpane. As long as your MacPro isn't moving around it won't ever come off. The SSD is supported from below with zero stress on the connector. You can stick the unused drive sled back in also so you don't lose it, it won't mess with your SSD.
You could even literally just click the drive in without the little tray holding it up but that would put a lot of stress on the connector. There are lot's of 2.5" to 3.5" sled adaptors out there, but this was free, quick, easy and has worked for about 6 months with no problem.