The PSU and GPU can be moved forward later so it's not wasted money. Cooler N764D should work for an overclock if you want to try that later,
BTW your question was useful to me. I overclock old Dells as a hobby, and I wasn't aware of this beast, Thanks. I have enough old parts lying around to make one of those scream.
CPUs to look at E8600 3.33GHz 2 core, Q9650/Q9550 4 core, X5470 3.33 4core (adapter required) X5460,X5440 confirmed
http://www.userbenchmark.com/System/Dell-Inspiron-545/1378#Benchmarks
Unlocked QX9650, or cheaper QX6850 4 cores, X6800 2 core. The BIOS remains locked. Must be overclocked from inside Windows. None of these are confirmed.
I did some more research on this and the Core2 extremes are rated at 130W. except the X6800. For some reason my old E520 supports these, but I;m not seeing them on the 545.