That's my thought as well - there's an E7300 going for $65:
Intel Core2 Duo E7300 2.66GHz 3MB Cache SLAPB CPU Processor | eBay
From the supported CPU list that would appear to be the fastest C2D it will take due to FSB limitations.
E1400 benchmark: 1115
E7300 benchmark: 1808
PassMark - CPU Benchmarks - List of Benchmarked CPUs