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Cisco's whitepaper's (attached) figure 1 (last page) recommends a 3945 for 150 Mbps WAN link. However, Cisco's recommendations are conservative, i.e. they try to insure you'll never have a performance issue using their recommendation. If you don't expect to routinely saturate the link, you can often do fine going a "size" down or sometimes even two "sizes" down. I.e. you might be okay with a 3925 or even a 2951.
Conversely if you think you may, in a year or two, need to increase your bandwidth, you might want to upsize.