Which Mac Pro do you have? I'm assuming the 2008 if it came with 2GB of RAM (presumably the 2x1GB factory modules).
Generally speaking, the 2008 model performs best with all of its DIMM sockets occupied (which matched pairs in each bank, of course). Take for example, a 16GB configuration. 8x2GB (all 8 sockets populated) would outperform a 4x4GB configuration. Although the 10GB configuration in question isn't on this list, this is a good chart Barefeats published for the 2008 MP:
http://www.barefeats.com/harper3.html
Basically, the more sockets occupied, the better.