you'll need to stick with a core2duo with a 1066mhz or lower fsb. the e6600 (and any e6x00, e6x20 series cpu) should work without issue, but an e4600 could be iffy since the BIOS may not recognize the cpu. the e7xxx series will probably not work at all.
I know the upgrade-ability of a dell is quite slim when it comes to hopping it up. I was wondering though, if you could upgrade the CPU without to much trouble. I have the diminsion 9200 with the Intel Core 2 Duo 6300(1.86GHZ), and want to upgrade to the 2.4 GHZ model. would there be any major issues that would prevent this from working smoothly?