the rx sensitivity is the same between 1800 and 900, the only difference is in the propagation. Usually, one assumes 10dB difference between 900 and 1800.
For instance, in urban area, one antenna transmits:
GSM900 at 45dBm
DCS1800 at 45dBm
at 1km, a MS measures:
GSM900 = -80dBm
DCS1800 = -90dBm
(this is just an example)
a MS is supposed to be able to understand a GSM900 and DCS1800 signal down to -102dBm at least.