They lack the ability to lock the PCI-E freq. This typically limits them to a small OC..but if you just use a PCI vid card and set the PCI-E to run in sync with the FSB..you can get some decent FSB out of them.
945's have some pretty tight NB latency too..making it difficult to OC with higher speed memory, and most mobo's do not have enough NBv to supply the NB at the higher FSB's/RAM speeds.
If you need a good stand-by, or occasional bencher..they work just fine though. [Although a cheap 965 like a DFI Infinty would be a much better choice]