Actually this Dell Optiplex 745 can support a Intel Core 2 Quad Q6600 or and Q6700. I own one of these machines with a Q6600 in it with Windows 7 and 8GB of RAM and it runs great!
I upgraded to the core 2 6700 and it improved a little, not much. Still tends to stull when i load up on with 6 to 9 windows open. Which I guess is normal.
Question, i thought 4 GB of RAM was the realistic limit running a 32 bit system? Does all that RAM make a difference?