Well... your CPU's can be replaced but you're already at the top of the line for Harpertown. (Unless you want an extra 200mhz, in which case you can go for 2 Xeon X5492 @3.4Ghz)
The new Nehalem chips won't work due to socket/architecture changes.
You can upgrade to 32gb of Ram. (8 x 4Gb chips)
At the moment the 8800gt and 3870 are your only options for *upgrading* your graphics. (Or the Quadro FX 5600 if you need Professional Graphics)
You can also add a RAID configuration, either SATA or SAS (though SAS will cost you the drives plus the Apple Raid Card whereas SATA can be made into RAID 0, 1, or 1+0 (sometimes called 10) through Disk Utility)
You could add a Blu-Ray Drive since there are 2 extra SATA connectors on board. It does you no good (really) in OS X and those 2 extra connectors do NOT show up under bootcamp so this is kind of a wash...
Probably the best upgrade you could do would be either Solid State Drives or Velociraptors from WD. (Or a graphics upgrade if you have a 2600xt)
Ram is always good but it depends what you do?
What are your complete specs?